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16 RepRoduction the uterus lining breaks down. The cells, along with menstrual period is one of the signs of puberty in blood are passed out of the vagina. This is called girls. After menstruation, the uterus lining starts to a menstrual period. The appearance of the first re-form and another ovum starts to mature. Hormones and the menstrual cycle progesterone, which affects the uterus lining in the At the start of the cycle, the lining of the uterus wall same way as the oestrogens, making it grow thicker has broken down (menstruation). As each follicle and produce more blood vessels. in the ovaries develops, the amount of oestrogens produced by the ovary increases. The oestrogens act If the ovum is fertilised, the corpus luteum on the uterus and cause its lining to become thicker continues to release progesterone and so keeps the and develop more blood vessels. These are changes uterus in a state suitable for implantation. If the ovum that help an early embryo to implant. is not fertilised, the corpus luteum stops producing progesterone. As a result, the thickened lining of the Two hormones, produced by the pituitary gland uterus breaks down and loses blood, which escapes at the base of the brain, promote ovulation. The through the cervix and vagina. The events in the hormones are follicle-stimulating hormone (FSH) menstrual cycle are shown in Figure 16.58. and luteinising hormone, or lutropin (LH). They act on a ripe follicle and stimulate maturation and Menopause release of the ovum. Between the ages of 40 and 55, the ovaries cease to release ova or produce hormones. As a consequence, Once the ovum has been released, the follicle that menstrual periods cease, the woman can no longer produced it develops into a solid body called the have children, and sexual desire is gradually reduced. corpus luteum. This produces a hormone called menstruation uterus lining thickens ready to breaks down if menstruation receive embryo no implantation takes place follicle maturing ovulation corpus luteum developing corpus luteum breaks down FSH and LH prog oestrogens esterone menstruation menstruation DAYS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1 2 3 4 5 start of end of copulation could menstruation menstruation result in fertilisation Figure 16.58 The menstrual cycle 242

Methods of birth control in humans ●● Methods of birth Artificial methods of family planning control in humans Barrier methods As little as 4 weeks after giving birth, it is possible, Sheath or condom though unlikely, that a woman may conceive again. A thin rubber sheath is placed on the erect penis Frequent breastfeeding may reduce the chances of before sexual intercourse. The sheath traps the sperm conception. Nevertheless, it would be possible to and prevents them from reaching the uterus. It also have children at about 1-year intervals. Most people prevents the transmission of sexually transmitted do not want, or cannot afford, to have as many infections (STIs). children as this. All human communities, therefore, practise some form of birth control to space out Diaphragm births and limit the size of the family. A thin rubber disc, placed in the vagina before intercourse, covers the cervix and stops sperm Natural methods of family planning entering the uterus. Condoms and diaphragms, used in conjunction with chemicals that immobilise sperm, Abstinence are about 95% effective. However, a diaphragm does This is the most obvious way of preventing a not prevent the risk of transmission of STIs. pregnancy. This involves a couple avoiding sexual intercourse. In this way, sperm cannot come into Femidom contact with an egg and fertilisation cannot happen. This is a female condom. It is a sheath or pouch, made of polyurethane or rubber, with a flexible ring at each Monitoring body temperature end. The ring at the closed end of the sheath is inserted If it were possible to know exactly when ovulation into the vagina to hold the femidom in place. The ring occurred, intercourse could be avoided for 3–4 days at the open end is placed outside the vagina. During before and 1 day after ovulation. At the moment, sexual intercourse, semen is trapped inside the femidom. however, there is no simple, reliable way to recognise A femidom reduces the risk of infection by STIs. ovulation, though it is usually 12–16 days before the onset of the next menstrual period. By keeping careful Chemical methods records of the intervals between menstrual periods, it Spermicides is possible to calculate a potentially fertile period of Spermicides are chemicals which, though harmless about 10 days in mid-cycle, when sexual intercourse to the tissues, can kill or immobilise sperm. The should be avoided if children are not wanted. spermicide, in the form of a cream, gel or foam, is placed in the vagina. On their own, spermicides are On its own, this method is not very reliable but not very reliable but, in conjunction with condoms or there are some physiological clues that help to make diaphragms, they are effective. it more accurate. During or soon after ovulation, Intra-uterine device (IUD) a woman’s temperature rises by about 0.5 °C. It is A small T-shaped plastic and copper device, also reasonable to assume that 1 day after the temperature known as a coil, can be inserted by a doctor or nurse returns to normal, a woman will be infertile. into the wall of the uterus, where it probably prevents implantation of a fertilised ovum. It is about 98% Cervical mucus effective but there is a small risk of developing uterine Another clue comes from the type of mucus secreted infections, and it does not protect against STIs. by the cervix and lining of the vagina. As the time Intra-uterine system (IUS) for ovulation approaches, the mucus becomes more This is similar to an IUD; is T-shaped and releases fluid. Women can learn to detect these changes and the hormone progesterone slowly over a long period so calculate their fertile period. of time (up to 5 years). The hormone prevents ovulation. An IUS does not protect against STIs. By combining the ‘calendar’, ‘temperature’ and Contraceptive pill ‘mucus’ methods, it is possible to achieve about 80% The pill contains chemicals, which have the same ‘success’, i.e. only 20% unplanned pregnancies. Highly effect on the body as the hormones oestrogen and motivated couples may achieve better rates of success and, of course, it is a very helpful way of finding the fertile period for couples who do want to conceive. 243

16 Reproduction progesterone. When mixed in suitable proportions thins the lining of the uterus, making it unsuitable these hormones suppress ovulation and so prevent for implantation of an embryo. It does not protect conception. The pills need to be taken each day for against STIs. the 21 days between menstrual periods. Surgical methods Male sterilisation – vasectomy There are many varieties of contraceptive This is a simple and safe surgical operation in which pill in which the relative proportions of the man’s sperm ducts are cut and the ends sealed. oestrogen- and progesterone-like chemicals This means that his semen contains the secretions of vary. They are 99% effective, but long-term use the prostate gland and seminal vesicle but no sperm, of some types may increase the risk of cancer of so cannot fertilise an ovum. Sexual desire, erection, the breast and cervix. The pill does not protect copulation and ejaculation are quite unaffected. against STIs. The testis continues to produce sperm and Contraceptive implant testosterone. The sperm are removed by white cells as This is a small plastic tube about 4 cm long, fast as they form. The testosterone ensures that there which is inserted under the skin of the upper is no loss of masculinity. arm of a woman by a doctor or nurse. Once in place it slowly releases the hormone progesterone, The sperm ducts can be rejoined by surgery but preventing pregnancy. It lasts for about 3 years. this is not always successful. It does not protect against STIs, but has more Female sterilisation – laparotomy than a 99% success rate in preventing A woman may be sterilised by an operation in which pregnancy. her oviducts are tied, blocked or cut. The ovaries are Contraceptive injection unaffected. Sexual desire and menstruation continue This injection, given to women, contains as before, but sperm can no longer reach the ova. progesterone and stays effective for between 8 and Ova are released, but break down in the upper part of 12 weeks. It works by thickening the mucus in the the oviduct. cervix, stopping sperm reaching an egg. It also The operation cannot usually be reversed. The use of hormones in fertility and contraception treatments insemination (AI). This involves injecting semen through a tube into the top of the uterus. In some Infertility cases, the husband’s semen can be used but, more About 85–90% of couples trying for a baby achieve often, the semen is supplied by an anonymous pregnancy within a year. Those that do not may be donor. sub-fertile or infertile. Female infertility is usually caused by a failure to ovulate or a blockage or With AI, the woman has the satisfaction of bearing distortion of the oviducts. The latter can often be her child rather than adopting, and 50% of the corrected by surgery. child’s genes are from the mother. It also allows a Using hormones to improve fertility couple to have a baby that is biologically theirs if the Failure to produce ova can be treated with fertility man is infertile. drugs. These drugs are similar to hormones and act by increasing the levels of FSH and LH. Apart from religious or moral objections, the Administration of the drug is timed to promote disadvantages are that the child can never know his ovulation to coincide with copulation. or her father and there may be legal problems about Artificial insemination (AI) the legitimacy of the child in some countries. Male infertility is caused by an inadequate quantity In vitro fertilisation of sperm in the semen or by sperm that are ‘In vitro’ means literally ‘in glass’ or, in other words, insufficiently mobile to reach the oviducts. There the fertilisation is allowed to take place in laboratory are few effective treatments for this condition, glassware (hence the term ‘test-tube babies’). This but pregnancy may be achieved by artificial technique may be employed where surgery cannot be used to repair blocked oviducts. In vitro fertilisation has received considerable publicity since the first ‘test-tube’ baby was born 244

Sexually transmitted infections (STIs) in 1978. The woman may be given fertility drugs, prevents the release of FSH. If FSH is blocked, no which cause her ovaries to release several mature further ova are matured. The uterus lining needs ova simultaneously. These ova are then collected by to be thick to allow successful implantation of an laparoscopy, i.e. they are sucked up in a fine tube embryo. inserted through the abdominal wall. The ova are then mixed with the husband’s seminal fluid and Progesterone maintains the thickness of the watched under the microscope to see if cell division uterine lining. It also inhibits the secretion of takes place. (Figure 16.50 is a photograph of such an luteinising hormone (LH), which is responsible ‘in vitro’ fertilised ovum.) for ovulation. If LH is suppressed, ovulation cannot happen, so there are no ova to be One or more of the dividing zygotes are then fertilised. introduced to the woman’s uterus by means of a tube inserted through the cervix. Usually, only Because of the roles of oestrogen and one (or none) of the zygotes develops, though progesterone, they are used, singly or in occasionally there are multiple births. combination, in a range of contraceptive methods. The success rate for in vitro fertilisation is between S ocial implications of contraception and fertility 12 and 40% depending on how many embryos are treatments transplanted. However, new research using time- Some religions are against any artificial forms of lapse photography of the developing IVF embryos contraception and actively discourage the use of during the first few days of life could raise the success contraceptives such as the sheath and femidom. rate to up to 78%. It could also reduce the cost from However, these are important in the prevention of between £5000 and £10 000 for each treatment transmission of STDs in addition to their role as cycle to £750 in Britain. The photographs are used contraceptives. to select the best embryos, based on their early development. Fertility treatments such as in vitro fertilisation are controversial because of the ‘spare’ embryos that Using hormones for contraception are created and not returned to the uterus. Some Oestrogen and progesterone control important people believe that since these embryos are potential events in the menstrual cycle. human beings, they should not be destroyed or used for research. In some cases the ‘spare’ embryos have Oestrogen encourages the re-growth of the been frozen and used later if the first transplants did lining of the uterus wall after a period and not work. ●● Sexually transmitted other people, however. It is not known for certain infections (STIs) what proportion of HIV carriers will eventually develop AIDS: perhaps 30–50%, or more. Key definition A sexually transmitted infection is an infection that is HIV is transmitted by direct infection of the blood. Drug users who share needles contaminated with transmitted via body fluids through sexual contact. infected blood run a high risk of the disease. It can also be transmitted sexually, both between men and AIDS and HIV women and, especially, between homosexual men who practise anal intercourse. Prostitutes, who have The initials of AIDS stand for acquired immune many sexual partners, are at risk of being infected deficiency syndrome. (A ‘syndrome’ is a pattern if they have sex without using condoms and are, of symptoms associated with a particular disease.) therefore, a potential source of HIV to others. The virus that causes AIDS is the human immunodeficiency virus (HIV). Haemophiliacs have also fallen victim to AIDS. Haemophiliacs have to inject themselves with a blood After a person has been infected, years may pass product that contains a clotting factor. Before the before symptoms develop. So people may carry the risks were recognised, infected carriers sometimes virus yet not show any symptoms. They can still infect donated blood, which was used to produce the clotting factor. 245

16 RepRoduction Babies born to HIV carriers may become infected These are good reasons, among many others, for with HIV, either in the uterus or during birth or being faithful to one partner. from the mother’s milk. The rate of infection varies from about 40% in parts of Africa to 14% in Europe. The risk of catching a sexually transmitted disease If the mother is given drug therapy during labour and can be greatly reduced if the man uses a condom or the baby within 3 days, this method of transmission is if a woman uses a femidom. These act as barriers to reduced. bacteria or viruses. There is no evidence to suggest that the disease can If a person suspects that he or she has caught be passed on by droplets (Chapter 10), by saliva or by a sexually transmitted disease, treatment must normal everyday contact. be sought at once. Information about treatment can be obtained by phoning one of the numbers When AIDS first appeared, there were no effective listed under ‘Venereal Disease’ or ‘Health drugs. Today, there is a range of drugs that can be Information Service’ in the telephone directory. given separately or as a ‘cocktail’, which slow the Treatment is always confidential. The patients progress of the disease. Research to find a vaccine and must, however, ensure that anyone they have had more effective drugs is ongoing. sexual contact with also gets treatment. There is no point in one partner being cured if the other is still There is a range of blood tests designed to detect infected. HIV infection. These tests do not detect the virus but do indicate whether antibodies to the virus are in STIs that are caused by a bacterium, such as the blood. If HIV antibodies are present, the person syphilis and gonorrhoea, can be treated with is said to be HIV positive. The tests vary in their antibiotics if the symptoms are recognised early reliability and some are too expensive for widespread enough. However, HIV is viral so antibiotics are not use. The American Food and Drug Administration effective. claims a 99.8% accuracy, but this figure is disputed. The effects of HIV on the immune Control of the spread of STIs system The best way to avoid sexually transmitted infections HIV attacks certain kinds of lymphocyte (see is to avoid having sexual intercourse with an infected ‘Blood’ in Chapter 9), so the number of these person. However, the symptoms of the disease are cells in the body decreases. Lymphocytes produce often not obvious and it is difficult to recognise an antibodies against infections. If the body cannot infected individual. So the disease is avoided by not respond to infections through the immune system, having sexual intercourse with a person who might it becomes vulnerable to pathogens that might not have the disease. Such persons are: otherwise be life-threatening. As a result, the patient ● prostitutes who offer sexual intercourse for money has little or no resistance to a wide range of diseases ● people who are known to have had sexual such as influenza, pneumonia, blood disorders, skin cancer or damage to the nervous system, which the relationships with many others body cannot resist. ● casual acquaintances whose background and past sexual activities are not known. Questions 6 Put the following events in the correct order for pollination Core in a lupin plant: 1 Plants can often be propagated from stems but rarely from A Bee gets dusted with pollen. B Pollen is deposited on stigma. roots. What features of shoots account for this difference? C Bee visits older flower. 2 The plants that survive a heath fire are often those that have D Bee visits young flower. E Anthers split open. a rhizome (e.g. ferns). Suggest a reason why this is so. 7 What are the functions in a seed of: 3 Working from outside to inside, list the parts of a bisexual a the radicle b the plumule flower. c the cotyledons? 4 What features of flowers might attract insects? 5 Which part of a flower becomes: a the seed b the fruit? 246

Sexually transmitted infections (Stis) 8 During germination of the broad bean, how are the 22 In what ways will the composition of the blood in the following parts protected from damage as they are forced umbilical vein differ from that in the umbilical artery? 23 An embryo is surrounded with fluid, its lungs are filled through the soil: a the plumule with fluid and it cannot breathe. Why doesn’t it suffocate? b the radicle? 24 If a mother gives birth to twin boys, does this mean that 9 List all the possible purposes for which a growing seedling they are identical twins? Explain. might use the food stored in its cotyledons. 25 Study Figures 16.51 and 16.52. On each diagram the age 10 At what stage of development is a seedling able to stop and size of the developing embryo are stated. depending on the cotyledons for its food? a Copy and complete the following table: 11 What do you think are the advantages to a germinating Age/weeks Size/mm seed of having its radicle growing some time before the 0 0 2 shoot starts to grow? 5 12 a Describe the natural conditions in the soil that would be 8 10 most favourable for germination. 20 b How could a gardener try to create these conditions? 35 13 How do sperm differ from ova in their structure (see Figure b Use the data in your table to plot a graph to show the 16.39)? growth of the embryo. 14 List the structures, in the correct order, through which the Extended sperm must pass from the time they are produced in the 26 In what ways does asexual reproduction in Mucor differ testis, to the time they leave the urethra. from asexual reproduction in flowering plants? 15 What structures are shown in Figure 16.44, but are not 27 A gardener finds a new and attractive plant produced shown in Figure 16.43? as a result of a chance mutation. Should she attempt to 16 In what ways does a zygote differ from any other cell in produce more of the same plant by self-pollination or by vegetative propagation? Explain your reasoning. the body? 28 Which of the following do not play a part in asexual 17 If a woman starts ovulating at 13 years old and stops at 50: reproduction? mitosis, gametes, meiosis, cell division, chromosomes, a how many ova are likely to be released from her ovaries zygote b about how many of these are likely to be fertilised? 29 Revise asexual reproduction and then state how we exploit 18 List, in the correct order, the parts of the female the process of asexual reproduction in plants. 30 Which structures in a flower produce: reproductive system through which sperm must pass a the male gametes b the female gametes? before reaching and fertilising an ovum. 31 In not more than two sentences, distinguish between the 19 State exactly what happens at the moment of fertilisation. terms pollination and fertilisation. 20 Is fertilisation likely to occur if mating takes place: 32 In flowering plants: a can pollination occur without fertilisation a 2 days before ovulation b can fertilisation occur without pollination? b 2 days after ovulation? 33 Which parts of a tomato flower: a grow to form the fruit Explain your answers. b fall off after fertilisation 21 Draw up a table with three columns as shown below. In c remain attached to the fruit? 34 From the list of changes at puberty in girls, select those the first column write: that are related to childbearing and say what part you think they play. male reproductive organs 35 One of the first signs of pregnancy is that the menstrual periods stop. Explain why you would expect this. female reproductive organs male gamete female gamete place where fertilisation occurs zygote grows into Now complete the other two columns. Flowering plants Mammals male reproductive organs female reproductive organs male gamete, etc. 247

16 Reproduction Checklist Sexual reproduction in plants After studying Chapter 16 you should know and understand the • Flowers contain the reproductive organs of plants. following: • The stamens are the male organs. They produce pollen Asexual reproduction grains, which contain the male gamete. • The carpels are the female organs. They produce ovules, • Asexual reproduction is the process resulting in the production of genetically identical offspring from one parent. which contain the female gamete and will form the seeds. • The flowers of most plant species contain male and female • Asexual reproduction occurs without gametes or fertilisation. • Fungi can reproduce asexually by single-celled spores. organs. A few species have unisexual flowers. • Many flowering plants reproduce asexually by vegetative • Brightly coloured petals attract insects, which pollinate the propagation. flower. • Plants reproduce asexually when some of their buds grow • Pollination is the transfer of pollen from the anthers of one into new plants. flower to the stigma of a flower on the same or another • The stolon of the strawberry plant is a horizontal stem plant. • Pollination may be carried out by insects or by the wind. that grows above the ground, takes root at the nodes and • Flowers that are pollinated by insects are usually brightly produces new plants. coloured and have nectar. • The couch grass rhizome is a horizontal stem that grows • Flowers that are pollinated by the wind are usually small and below the ground and sends up shoots from its nodes. green. Their stigmas and anthers hang outside the flower • Bulbs are condensed shoots with circular fleshy leaves. Bulb- where they are exposed to air movements. forming plants reproduce asexually from lateral buds. • Fertilisation occurs when a pollen tube grows from a pollen • Rhizomes, corms, bulbs and tap roots may store food, which grain into the ovary and up to an ovule. The pollen nucleus is used to accelerate early growth. passes down the tube and fuses with the ovule nucleus. • A clone is a population of organisms produced asexually from • After fertilisation, the ovary grows rapidly to become a fruit a single parent. and the ovules become seeds. • Whole plants can be produced from single cells or small • Germination is influenced by temperature and the amount of pieces of tissue. water and oxygen available. • Artificial propagation from cuttings or grafts preserves the • Self-pollination is the transfer of pollen grains from the desirable characteristics of a crop plant. anther of a flower to the stigma of the same flower. • Vegetative propagation produces (genetically) identical • Cross-pollination is transfer of pollen grains from the individuals. anther of a flower to the stigma of a flower on a different plant of the same species. • Asexual reproduction keeps the characteristics of the organism the same from one generation to the next, but • Self-pollination and cross-pollination have implications to does not result in variation to cope with environmental a species. change. Sexual reproduction in humans Sexual reproduction • The male reproductive cells (gametes) are sperm. They are • Sexual reproduction is the process involving the fusion of the produced in the testes and expelled through the urethra and nuclei of two gametes (sex cells) to form a zygote and the penis during mating. production of offspring that are genetically different from each other. • The female reproductive cells (gametes) are ova (eggs). They are produced in the ovaries. One is released each month. If • The male gamete is small and mobile. The female gamete is sperm are present, the ovum may be fertilised as it passes larger and not often mobile. down the oviduct to the uterus. • The male gamete of an animal is a sperm. The male gamete • Fertilisation happens when a sperm enters an ovum and the of a flowering plant is the pollen nucleus. sperm and egg nuclei join up (fuse). • The female gamete of an animal is an ovum. The female • The fertilised ovum (zygote) divides into many cells and gamete of a flowering plant is an egg cell in an ovule. becomes embedded in the lining of the uterus. Here it grows into an embryo. • Fertilisation is the fusion of gamete nuclei. • The embryo gets its food and oxygen from its mother. • The nuclei of gametes are haploid and the nucleus of the • The embryo’s blood is pumped through blood vessels in zygote is diploid. the umbilical cord to the placenta, which is attached to • There are advantages and disadvantages of sexual the uterus lining. The embryo’s blood comes very close to reproduction to a species. the mother’s blood so that food and oxygen can be picked up and carbon dioxide and nitrogenous waste can be • There are advantages and disadvantages of sexual got rid of. reproduction in crop production. 248

Sexually transmitted infections (STIs) • Good ante-natal care, in the form of special dietary needs Methods of birth control in humans and maintaining good health, is needed to support the mother and her fetus. • There are effective ways of spacing births and limiting the size of a family. These include natural, chemical, barrier and • When the embryo is fully grown, it is pushed out of the surgical methods. uterus through the vagina by contractions of the uterus and abdomen. • Hormones can be used to control fertility, including contraception and promoting egg-cell development. • Twins may result from two ova being fertilised at the same time or from a zygote forming two embryos. • Female infertility may be relieved by surgery, fertility drugs or in vitro fertilisation. • Eggs and sperm are different in size, structure, mobility and numbers produced. • Male infertility can be by-passed by artificial insemination. • There are social implications of using hormones in • Sperm and eggs have special features to adapt them for their functions. contraception and for increasing the chances of pregnancy. • The placenta and umbilical cord are involved in exchange of materials between the mother and fetus. Some toxins Sexually transmitted infections (STIs) and viruses can also be passed across and affect the fetus. • A sexually transmitted infection is an infection transmitted via • Human milk and breastfeeding are best for babies. bodily fluids through sexual contact. Sex hormones in humans • HIV is an example of an STI. • HIV can be transmitted in a number of ways. • At puberty, the testes and ovaries start to produce mature • The spread of HIV can be controlled. gametes and the secondary sexual characteristics develop. • HIV infection may lead to AIDS. • Each month, the uterus lining thickens up in readiness to • HIV affects the immune system by reducing the number receive a fertilised ovum. If an ovum is not fertilised, the of lymphocytes and decreasing the ability to produce lining and some blood are lost through the vagina. This is antibodies. menstruation. • Oestrogen and progesterone are secreted by endocrine glands. • The release of ova and the development of an embryo are under the control of hormones like oestrogen, progesterone, follicle-stimulating hormone and luteinising hormone. 249

17 Inheritance Inheritance Meiosis Define inheritance Define meiosis Role of meiosis Chromosomes, genes and proteins Define chromosome and gene The process of mitosis Inheritance of sex in humans The function of chromosomes Stem cells Genetic code for proteins Gamete production and chromosomes Role of DNA in cell function Meiosis How a protein is made Gene expression Monohybrid inheritance Define haploid nucleus, diploid nucleus Define allele, genotype, phenotype, homozygous, heterozygous, Diploid cells dominant, recessive Use of genetic diagrams and Punnett squares Mitosis Define mitosis Use of test crosses Role of mitosis Co-dominance and incomplete dominance Duplication and separation of chromosomes Define sex-linked characteristic Colour blindness Genetic crosses involving co-dominance and sex linkage ●● Inheritance Key definition centromere Inheritance is the transmission of genetic information from generation to generation. We often talk about people inheriting certain chromatid characteristics: ‘Nathan has inherited his father’s curly hair’, or ‘Fatima has inherited her mother’s brown gene for hair eyes’. We expect tall parents to have tall children. The colour inheritance of such characteristics is called heredity and the branch of biology that studies how heredity Figure 17.1 Structure of a chromosome works is called genetics. ●● Chromosomes, genes A human body (somatic) cell nucleus contains and proteins 46 chromosomes. These are difficult to distinguish when packed inside the nucleus, so scientists separate Key definitions them and arrange them according to size and A chromosome is a thread of DNA, made up of a string of appearance. The outcome is called a karyotype (Figure 17.2). There are pairs of chromosomes. The genes. only pair that do not necessarily match is chromosome A gene is a length of DNA that codes for a protein. pair 23: the ‘sex chromosomes’. The Y chromosome is much smaller than the X chromosome. Inside a nucleus are thread-like structures called chromosomes which can be seen most clearly at the The inheritance of sex time when the cell is dividing. Each chromosome has certain characteristics when ready to divide: there Whether you are a male or female depends on the are two chromatids, joined at one point called a pair of chromosomes called the ‘sex chromosomes’. centromere (Figure 17.1). Each chromatid is a string of In females, the two sex chromosomes, called genes, coding for the person’s characteristics. The other the X chromosomes, are the same size as each chromatid carries the same genes in the same order. other. In males, the two sex chromosomes are of different sizes. One corresponds to the female sex 250

Chromosomes, genes and proteins X chromosome and 50% getting a Y chromosome (Figure 17.3). If an X sperm fertilises the ovum, the zygote will be XX and will grow into a girl. If a Y sperm fertilises the ovum, the zygote will be XY and will develop into a boy. There is an equal chance of an X or Y chromosome fertilising an ovum, so the numbers of girl and boy babies are more or less the same. Figure 17.4 shows how sex is inherited. parents male female Figure 17.2  Human karyotype sex chromosomes XY × XX of parents chromosomes and is called the X chromosome. The gametes (sex cells) XY × XX other is smaller and is called the Y chromosome. So the female cells contain XX and male cells contain XY. sex chromosomes XX XX XY XY of children A process called meiosis takes place in the female’s ovary. It makes gametes: sex cells, which have half the sex of female female male male normal number of chromosomes. During the process, children each ovum receives one of the X chromosomes, The ratio is 1 female : 1 male so all the ova are the same for this. Meiosis in the male’s testes results in 50% of the sperms getting an Figure 17.4  Determination of sex FEMALE MALE XX XY ovum- sperm- producing producing cell cell meiosis meiosis X X X Y ova sperms all ova will contain one X chromosome half the sperms will contain an Figure 17.3  Determination of sex. Note that: X chromosome and half will carry (i) only the X and Y chromosomes are shown a Y chromosome (ii) details of meiosis have been omitted (iii) in fact, four gametes are produced in each case, but two are sufficient to show the distribution of X and Y chromosomes 251

17 Inheritance The genetic code The chemical reactions that take place in a cell determine what sort of a cell it is and what its The structure of DNA has already been described in functions are. These chemical reactions are, in turn, Chapter 4. controlled by enzymes. Enzymes are proteins. It follows, therefore, that the genetic code of DNA, in Each nucleotide carries one of four bases (A, T, determining which proteins, particularly enzymes, C or G). A string of nucleotides therefore holds are produced in a cell, also determines the cell’s a sequence of bases. This sequence forms a code, structure and function. In this way, the genes also which instructs the cell to make particular proteins. determine the structure and function of the whole Proteins are made from amino acids linked together organism. (Chapter 4). The type and sequence of the amino acids joined together will determine the kind of Other proteins coded for in DNA include protein formed. For example, one protein molecule antibodies and the receptors for neurotransmitters may start with the sequence alanine–glycine–glycine (see details of synapses in Chapter 14). .... A different protein may start glycine–serine– alanine .... The manufacture of proteins in cells It is the sequence of bases in the DNA molecule that decides which amino acids are used and in DNA molecules remain in the nucleus, but the which order they are joined. Each group of three proteins they carry the codes for are needed bases stands for one amino acid, e.g. the triplet of elsewhere in the cell. A molecule called messenger bases CGA specifies the amino acid alanine, the RNA (mRNA) is used to transfer the information base triplet CAT specifies the amino acid valine, and from the nucleus. It is much smaller than a DNA the triplet CCA stands for glycine. The tri-peptide molecule and is made up of only one strand. Another valine–glycine–alanine is specified by the DNA code difference is that mRNA molecules contain slightly CAT–CCA–CGA (Figure 17.5). different bases (A, C, G and U). Base U is uracil. It attaches to the DNA base A. A gene, then, is a sequence of triplets of the four bases, which specifies an entire protein. Insulin is a To pass on the protein code, the double helix small protein with only 51 amino acids. A sequence of DNA (see Figure 4.12) unwinds to expose the of 153 (i.e. 3 × 51) bases in the DNA molecule chain of bases. One strand acts as template. A would constitute the gene that makes an islet cell messenger RNA molecule is formed along part in the pancreas produce insulin. Most proteins are of this strand, made up of a chain of nucleotides much larger than this and most genes contain a with complementary bases to a section of the DNA thousand or more bases. strand (Figure 17.6). The mRNA molecule carrying the protein code then passes out of the nucleus, The DNA base sequence . . . determines . . . the sequence of amino through a nuclear pore in the membrane. Once acids in a peptide in the cytoplasm it attaches itself to a ribosome. Ribosomes make proteins. The mRNA molecule C instructs the ribosome to put together a chain of A valine amino acids in a specific sequence, thus making a T protein. Other mRNA molecules will carry codes for C different proteins. C glycine A Some proteins are made up of a relatively small C number of amino acids. As stated, insulin is a chain G alanine of 51 amino acids. On the mRNA molecule each A amino acid is coded by a sequence of three bases (a triplet), so the mRNA molecule coding for insulin Figure 17.5  The genetic code (triplet code) will contain 153 bases. Other protein molecules are much bigger: haemoglobin in red blood cells is made of 574 amino acids. 252

Chromosomes, genes and proteins key GC GC (they come from a diploid cell). Because the A – adenine AT chromosomes are in pairs, the diploid number is T – thymine TA AT always an even number. The karyotype of a sperm cell C – cytosine would show 23 single chromosomes (they come from bases CG TA a haploid cell). The sex chromosome would be either AT X or Y. The chromosomes have different shapes and G – guanine AT GC sizes and can be recognised by a trained observer. U – uracil GC TA There is a fixed number of chromosomes in each species. Human body cells each contain A TA 46 chromosomes, mouse cells contain 40 and garden pea cells 14 (see also Figure 17.7). C mRNA GC G nucleotides The number of chromosomes in a species is GC the same in all of its body cells. There are U 46 chromosomes in each of your liver cells, in every TA nerve cell, skin cell and so on. GC GC The chromosomes are always in pairs (Figure 17.7), e.g. two long ones, two short ones, TA TA A two medium ones. This is because when the zygote AU T is formed, one of each pair comes from the male GC gamete and one from the female gamete. Your CG G 46 chromosomes consist of 23 from your mother TA and 23 from your father. AT TA A The chromosomes of each pair are called AU T homologous chromosomes. In Figure 17.18(b), the C G AU T two long chromosomes form one homologous pair A GC C and the two short chromosomes form another. T T CG A T G A C G C A G G C G C (a) The DNA helix unwinds: U the strands separate, exposing the bases. (b) mRNA nucleotides attach to the exposed DNA bases to form a mRNA molecule. Figure 17.6  Formation of messenger RNA Gene expression kangaroo (12) human (46) Body cells do not all have the same requirements for proteins. For example, the function of some cells in the stomach is to make the protein pepsin (see ‘Chemical digestion’ in Chapter 7). Bone marrow cells make the protein haemoglobin, but do not need digestive enzymes. Specialised cells all contain the same genes in their nuclei, but only the genes needed to code for specific proteins are switched on (expressed). This enables the cell to make only the proteins it needs to fulfil its function. Key definitions A haploid nucleus is a nucleus containing a single set of unpaired chromosomes present, for example, in sperm and egg cells. A diploid nucleus is a nucleus containing two sets of chromosomes present, for example, in body cells. Number of chromosomes domestic fowl (36) fruit fly (8) Figure 17.2 is a karyotype of a human body cell Figure 17.7  Chromosomes of different species. Note that the because there are 23 pairs of chromosomes present chromosomes are always in pairs. 253

17 InherItanCe ●● Mitosis Cell division Key definitions When plants and animals grow, their cells increase Mitosis is nuclear division giving rise to genetically in number by dividing. Typical growing regions are the ends of bones, layers of cells in the skin, root identical cells. tips and buds (Figure 17.11). Each cell divides to produce two daughter cells. Both daughter cells may Genetics is the study of inheritance. It can be used divide again, but usually one of the cells grows and to forecast what sorts of offspring are likely to be changes its shape and structure and becomes adapted produced when plants or animals reproduce sexually. to do one particular job – in other words, it becomes What will be the eye colour of children whose mother specialised (Figure 17.8). At the same time it loses has blue eyes and whose father has brown eyes? Will its ability to divide any more. The other cell is still a mating between a black mouse and a white mouse able to divide and so continue the growth of the produce grey mice, black-and-white mice or some tissue. Growth is, therefore, the result of cell division, black and some white mice? followed by cell enlargement and, in many cases, cell specialisation. To understand the method of inheritance, we need to look once again at the process of cell becomes sexual reproduction and fertilisation. In sexual specialised reproduction, a new organism starts life as a single cell called a zygote (Chapter 16). This means that cell division you started from a single cell. Although you were supplied with oxygen and food in the uterus, all cell division cell retains your tissues and organs were produced by cell power to division from this one cell. So, the ‘instructions’ that dictated which cells were to become liver divide or muscle or bone must all have been present in this first cell. The instructions that decided Figure 17.8 Cell division and specialisation. Cells that retain the ability that you should be tall or short, dark or fair, male to divide are sometimes called stem cells. or female must also have been present in the zygote. The process of cell division in an animal cell is shown in Figure 17.9. The events in a plant cell The process of mitosis is important in growth. are shown in Figures 17.10 and 17.11. Because of We all started off as a single cell (a zygote). That cell the cell wall, the cytoplasm cannot simply pinch off divided into two cells, then four and so on, to create in the middle, and a new wall has to be laid down the organism we are now, made up of millions of between the two daughter cells. Also a new vacuole cells. Cells have a finite life: they wear out or become has to form. damaged, so they need to be replaced constantly. The processes of growth, repair and replacement Organelles such as mitochondria and chloroplasts of cells all rely on mitosis. Organisms that reproduce are able to divide and are shared more or less equally asexually (see Chapter 16) also use mitosis to create between the daughter cells at cell division. more cells. (a) Animal cell about to (b) The nucleus divides first. (c) The daughter nuclei separate (d) Two cells are formed – one divide. and the cytoplasm pinches may keep the ability to off between the nuclei. divide, and the other may become specialised. Figure 17.9 Cell division in an animal cell 254

Meiosis (a) A plant cell (b) The nucleus divides (c) The cytoplasm adds layers of (d) The vacuoles join up to form one about to divide first. A new cell wall cellulose on each side of the new vacuole. This takes in water and makes has a large develops and cell wall. Vacuoles form in the the cell bigger. The other cell will divide nucleus and separates the two cytoplasm of one cell. again. no vacuole. cells. Figure 17.10 Cell division in a plant cell Making the squash preparation ■ Squash the softened, stained root tips by lightly tapping on the coverslip with a pencil: hold the pencil vertically and let it slip through the fingers to strike the coverslip (Figure 17.12). ■ The root tip will spread out as a pink mass on the slide; the cells will separate and the nuclei, many of them with chromosomes in various stages of mitosis (because the root tip is a region of rapid cell division), can be seen under the high power of the microscope (×400). Figure 17.11 Cell division in an onion root tip (×250). The nuclei are Figure 17.12 Tap the coverslip gently to squash the tissue stained blue. Most of the cells have just completed cell division. Practical work Squash preparation of ●● Meiosis chromosomes using acetic orcein Key definitions Preparation of root tips Meiosis is nuclear division, which gives rise to cells that are ■ Support Allium cepa (onion) root tips over beakers or jars of water. genetically different. ■ Keep the onions in darkness for several days until the roots The process of meiosis takes place in the gonads growing into the water are 2–3 cm long. of animals (e.g. the testes and ovaries of mammals, ■ Cut off about 5 mm of the root tips and place them in a and the anthers and ovules of flowering plants). The cells formed are gametes (sperm and egg cells watch glass. in mammals; egg cells and pollen grain nuclei in ■ Cover the root tips with nine drops acetic orcein and one drop flowering plants). Gametes are different from other cells because they have half the normal number of molar hydrochloric acid. chromosomes (they are haploid). ■ Heat the watch glass gently over a very small Bunsen flame till the steam rises from the stain, but do not boil. ■ Leave the watch glass covered for at least 5 minutes. ■ Place one of the root tips on a clean slide, cover with 45% ethanoic (acetic) acid and cut away all but the terminal 1 mm. ■ Cover this root tip with a clean coverslip and make a squash preparation as described next. 255

17 InherItanCe The process of mitosis To understand how the ‘instructions’ are passed nucleus with two cell nuclear from cell to cell, we need to look in more detail at chromosomes membrane membrane what happens when the zygote divides and produces an organism consisting of thousands of cells. This (a) Just before the cell divides, (b) The chromosomes get type of cell division is called mitosis. It takes place chromosomes appear in the shorter and thicker. not only in a zygote but in all growing tissues. nucleus. When a cell is not dividing, there is very little two two fibres pull detailed structure to be seen in the nucleus even if it chromatids chromatids chromatids apart is treated with special dyes called stains. Just before cell division, however, a number of long, thread-like (c) Each chromosome is (d) The nuclear membrane structures appear in the nucleus and show up very now seen to consist of disappears and the clearly when the nucleus is stained (Figures 17.13 two chromatids. chromatids are pulled apart and 17.14). These thread-like structures are called to opposite ends of the cell. chromosomes. Although they are present in the nucleus all the time, they show up clearly only at cell nuclear membrane nucleus with two ‘daughter’ division because at this time they get shorter and forms chromosomes cell thicker. (e) A nuclear membrane forms (f) Cell division completed, Each chromosome duplicates itself and is seen round each set of chromatids, giving two ‘daughter’ cells, to be made up of two parallel strands, called and the cell starts to divide. each containing the same chromatids (Figure 17.1). When the nucleus number of chromosomes divides into two, one chromatid from each as the parent cell. chromosome goes into each daughter nucleus. The chromatids in each nucleus now become Figure 17.13 Mitosis. Only two chromosomes are shown. Three chromosomes and later they will make copies of of the stages described here are shown in Figure 17.14. themselves ready for the next cell division. The process of copying is called replication because each chromosome makes a replica (an exact copy) of itself. As Figure 17.13 is a simplified diagram of mitosis, only two chromosomes are shown, but there are always more than this. Human cells contain 46 chromosomes. Mitosis will be taking place in any part of a plant or animal that is producing new cells for growth or replacement. Bone marrow produces new blood cells by mitosis; the epidermal cells of the skin are replaced by mitotic divisions in the basal layer; new epithelial cells lining the alimentary canal are produced by mitosis; growth of muscle or bone in animals, and root, leaf, stem or fruit in plants, results from mitotic cell divisions. An exception to this occurs in the final stages of gamete production in the reproductive organs of plants and animals. The cell divisions that give rise to gametes are not mitotic but meiotic. Cells that are not involved in the production of gametes are called somatic cells. Mitosis takes place only in somatic cells. 256

Meiosis (b) (f) AA AA A A (d) BB BB B B CC C C H CC DD I DD D ED EE EE E F FF F G GG FF G H H I H GG I I HH II Figure 17.14  Mitosis in a root tip (×500). The letters refer to the stages (a) A chromosome (b) When the cell (c) Mitosis described in Figure 17.13. (The tissue has been squashed to separate builds upa divides, the original separates the the cells.) replica of and the replica are chromatids. Each itself. called chromatids. new cell getsa The function of chromosomes full set of genes. When a cell is not dividing, its chromosomes Figure 17.16  Replication. (A, B, C, etc. represent genes.) become very long and thin. Along the length of the chromosome is a series of chemical structures Which of the instructions are used depends on where called genes (Figure 17.15). The chemical that a cell finally ends up. The gene that causes brown forms the genes is called DNA (which is short eyes will have no effect in a stomach cell and the for deoxyribonucleic acid, Chapter 4). Each gene gene for making pepsin will not function in the cells controls some part of the chemistry of the cell. It is of the eye. So a gene’s chemical instructions are these genes that provide the ‘instructions’ mentioned carried out only in the correct situation. at the beginning of the chapter. For example, one gene may ‘instruct’ the cell to make the pigment The genes that produce a specific effect in a cell that is formed in the iris of brown eyes. On one (or whole organism) are said to be expressed. In the chromosome there will be a gene that causes the cells stomach lining, the gene for pepsin is expressed. The of the stomach to make the enzyme pepsin. When gene for melanin (the pigment in brown eyes) is not the chromosome replicates, it builds an exact replica expressed. of itself, gene by gene (Figure 17.16). When the chromatids separate at mitosis, each cell will receive a Stem cells full set of genes. In this way, the chemical instructions in the zygote are passed on to all cells of the body. All Recent developments in tissue culture have involved the chromosomes, all the genes and, therefore, all the stem cells. Stem cells are those cells in the body that instructions are faithfully reproduced by mitosis and have retained their power of division. Examples are passed on complete to all the cells. the basal cells of the skin (‘Homeostasis’ in Chapter 14), which keep dividing to make new skin cells, and chromosome cells in the red bone marrow, which constantly divide to produce the whole range of blood cells (‘Blood’ gene for in Chapter 9). eye colour In normal circumstances this type of stem cell can gene genes for produce only one type of tissue: epidermis, blood, hair colour muscle, nerves, etc. Even so, culture of these stem cells could lead to effective therapies by introducing genes for healthy stem cells into the body to take over the tallness function of diseased or defective cells. Figure 17.15  Relationship between chromosomes and genes. Cells taken from early embryos (embryonic The drawing does not represent real genes or a real chromosome. stem cells) can be induced to develop into almost There are probably thousands of genes on a chromosome. any kind of cell, but there are ethical objections to using human embryos for this purpose. However, it has recently been shown that, given the right 257

17 Inheritance conditions, brain stem cells can become muscle or 23 meiosis 46 blood cells, and liver cells have been cultured from 46 meiosis ovum- blood stem cells. Scientists have also succeeded in producing reprogramming skin cells to develop into other sperm- 23 23 cell types of cell, such as nerve cells. Bone marrow cells producing sperms ova (only are used routinely to treat patients with leukaemia cell one develops) (cancer of white blood cells). The use of adult 23 23 stem cells does not have the ethical problems of fertilisation embryonic stem cells, since cells that could become whole organisms are not being destroyed. 46 zygote Gamete production and 46 46 cell division chromosomes by mitosis The genes on the chromosomes carry the instructions 46 46 46 46 embryo that turn a single-cell zygote into a bird or a rabbit 46 46 46 46 or an oak tree. The zygote is formed at fertilisation, when a male gamete fuses with a female gamete. Each 46 46 46 46 gamete brings a set of chromosomes to the zygote. 46 46 46 46 The gametes, therefore, must each contain only half the diploid number of chromosomes, otherwise the Figure 17.17  Chromosomes in gamete production and fertilisation chromosome number would double each time an organism reproduced sexually. Each human sperm cell By now (Figure 17.18(d)), each chromosome is seen contains 23 chromosomes and each human ovum has to consist of two chromatids and there is a second 23 chromosomes. When the sperm and ovum fuse at division of the nucleus (Figure 17.18(e)), which fertilisation (Chapter 16), the diploid number of 46 separates the chromatids into four distinct nuclei (23 + 23) chromosomes is produced (Figure 17.17). (Figure 17.18(f)). The process of cell division that gives rise to This gives rise to four gametes, each with the gametes is different from mitosis because it results haploid number of chromosomes. In the anther of in the cells containing only half the diploid number a plant (Chapter 16), four haploid pollen grains of chromosomes. This number is called the haploid are produced when a pollen mother cell divides by number and the process of cell division that gives rise meiosis (Figure 17.19). In the testis of an animal, to gametes is called meiosis. meiosis of each sperm-producing cell forms four sperm. In the cells of the ovule of a flowering Meiosis takes place only in reproductive organs. plant or the ovary of a mammal, meiosis gives rise to only one mature female gamete. Four gametes Meiosis may be produced initially, but only one of them turns into an egg cell that can be fertilised. In a diploid cell that is going to divide and produce gametes, the chromosomes shorten and thicken as in As a result of meiosis and fertilisation, the mitosis. The pairs of homologous chromosomes, maternal and paternal chromosomes meet in e.g. the two long ones and the two short ones in different combinations in the zygotes. Consequently, Figure 17.18(b), lie alongside each other and, when the offspring will differ from their parents and from the nucleus divides for the first time, it is the each other in a variety of ways. chromosomes and not the chromatids that are separated. This results in only half the total number Asexually produced organisms (Chapter 16) show of chromosomes going to each daughter cell. In no such variation because they are produced by Figure 17.18(c), the diploid number of four mitosis and all their cells are identical to those of chromosomes is being reduced to two chromosomes their single parent. prior to the first cell division. 258

Monohybrid inheritance Table 17.1 compares meiosis and mitosis. Table 17.1  Mitosis and meiosis compared Mitosis occurs during cell division of somatic cells Meiosis a full set of chromosomes is passed on to each daughter cell; this is the occurs in the final stages of cell division leading to production of diploid number of chromosomes gametes the chromosomes and genes in each daughter cell are identical only half the chromosomes are passed on to the daughter cells, i.e. the if new organisms are produced by mitosis in asexual reproduction (e.g. haploid number of chromosomes bulbs, Chapter 16) they will all resemble each other and their parents; they are said to be ‘clones’ homologous chromosomes and their genes are randomly assorted between the gametes new organisms produced by meiosis in sexual reproduction will show variations from each other and from their parents (a) The chromosomes appear. (b) Homologous chromosomes ooToTooTTThhhhhnnrrrgggooeoeeaaasssssccnnneeeaahhiiirriiisssrnnneemmmoommrrrff’’’eeesssrrooodddmmmssmmooaaaooommrrrttttteeehhhhheeeeeeefffssrrrrrroooffaa;;;aammmppttttthhhpphheeettteeeehhhaabbbrreee..rrlll..uuueee (b) Hlieomaloolnoggsoidues echarcohmoothsoemr. es (a) (b) Hlieomaloolnoggsoidues echarcohmoothsoemr. es (a) lie alongside each other. ones are from the father. (c) The nuclear membrane (d) By now each chromosome Figure 17.19  Meiosis in an anther (×1000). The last division of meiosis dTcddTocochhhhhippiisssrrreeppaaaooooopppnnmmmsspppuuiioooeeettcceesssaaalloooeerrreeaammmsssnnrraaaeeeddmmnnnsssssdddeemmmoommcccffoooooottbbvvvrrrhheeerrrrreeaaeeeaaannsssccppppppeeeeaaaooollllrrrnnn..tttdddtttiiiooonnnggg (d) Bhyasnboewcoemacehtcwhorocmhroosmomateids. in the anther of a flower produces four pollen grains. (c) (d) Bhyasnboewcoemacehtcwhorocmhroosmomateids. (c) ●● Monohybrid inheritance has become two chromatids. Key definitions opposite ends of the cell. An allele is a version of a gene. Genotype is the genetic make-up of an organism in terms of (e) A second division takes (f) Four gametes are formed. cppApAchhlllaaassrreeooccceeeccmmootttaannooottddiisssddeeeddsspppii..vvaaaiirrrssaaaiiootttnneee tthakees EoccoEoEFFhhooaaarrriiirruucccgggoohhhrriiimmnnnggcccaaaoooooaalllnnnssmmnnnoottteeaaauuummttiiimmmnnneeessssssbbb..eeeoooaarrrrrnnneellloooyyyfffffoohhhrraaammlllfffeetttddhhh..eee the alleles present. (e) tthakees (f) Phenotype is the features of an organism. (e) the (f) Homozygous means having two identical alleles of a Figucrhero1m7.a1t8id  s.Meiosis chromosomes. particular gene e.g. TT, where T is tall. Note that two identical homozygous individuals that breed together will be pure-breeding. Heterozygous means having two different alleles of a particular gene e.g. Tt. Note that a heterozygous individual will not be pure breeding. An allele that is expressed if it is present is dominant. An allele that is only expressed when there is no dominant allele of the gene present is recessive. 259

17 Inheritance Alleles Single-factor inheritance The genes that occupy corresponding positions on Because it is impossible to follow the inheritance homologous chromosomes and control the same of the thousands of characteristics controlled by characteristic are called allelomorphic genes, or genes, it is usual to start with the study of a single alleles. The word ‘allelomorph ’ means ‘alternative gene that controls one characteristic. We have used form’. For example, there are two alternative forms eye colour as an example so far. Probably more of a gene for eye colour. One allele produces brown than one allele pair is involved, but the simplified eyes and one allele produces blue eyes. example will serve our purpose. It has already been explained how an allele for brown eyes from There are often more than two alleles of a gene. each parent results in the child having brown eyes. The human ABO blood groups are controlled by Suppose, however, that the mother has blue eyes three alleles, though only two of these can be present and the father brown eyes. The child might receive in one genotype. an allele for blue eyes from its mother and an allele for brown eyes from its father (Figure 17.21). If this Patterns of inheritance happens, the child will, in fact, have brown eyes. The allele for brown eyes is said to be dominant A knowledge of mitosis and meiosis allows us to to the allele for blue eyes. Although the allele for explain, at least to some extent, how heredity works. blue eyes is present in all the child’s cells, it is not The allele in a mother’s body cells that causes her expressed. It is said to be recessive to brown. to have brown eyes may be present on one of the chromosomes in each ovum she produces. If the Eye colour is a useful ‘model’ for explaining father’s sperm cell contains an allele for brown eyes inheritance but it is not wholly reliable because ‘blue’ on the corresponding chromosome, the zygote will eyes vary in colour and sometimes contain small receive an allele for brown eyes from each parent. amounts of brown pigment. These alleles will be reproduced by mitosis in all the embryo’s body cells and when the embryo’s eyes allele for brown eyes develop, the alleles will make the cells of the iris produce brown pigment (melanin) and the child B b allele for will have brown eyes. In a similar way, the child may blue eyes receive alleles for curly hair. sperm from ovum from Figure 17.20 shows this happening, but it does father mother not, of course, show all the other chromosomes with thousands of genes for producing the enzymes, making B different types of cell and all the other processes that control the development of the organism. b allele for curly hair allele for C C zygote brown eyes B B Figure 17.21  Combination of alleles in the zygote (only one sperm ovum chromosome is shown). The zygote has both alleles for eye colour; the child will have brown eyes. C This example illustrates the following important CB points: B l There is a pair of alleles for each characteristic, one allele from each parent. zygote Figure 17.20  Fertilisation. Fertilisation restores the diploid l Although the allele pairs control the same number of chromosomes and combines the alleles characteristic, e.g. eye colour, they may have from the mother and father. different effects. One tries to produce blue eyes, the other tries to produce brown eyes. l Often one allele is dominant over the other. 260

Monohybrid inheritance l The alleles of each pair are on corresponding not true-breeding because they may produce some chromosomes and occupy corresponding positions. white babies as well as black ones. The Bb rabbits are For example, in Figure 17.20 the alleles for eye called heterozygous (‘hetero-’ means ‘different’). colour are shown in the corresponding position on the two short chromosomes and the alleles for hair The black BB rabbits are homozygous dominant. curliness are in corresponding positions on the two The white bb rabbits are homozygous recessive. long chromosomes. In diagrams and explanations of heredity: BB black male Bb l alleles are represented by letters rabbits meiosis l alleles controlling the same characteristic are 50 % given the same letter, and meiosis sperms l the dominant allele is given the capital letter. have B B all sperms B B 50 % b For example, in rabbits, the dominant allele for black carry B have b fur is labelled B. The recessive allele for white fur is labelled b to show that it corresponds to B for (a) true-breeding (b) not true-breeding black fur. If it were labelled w, we would not see any Figure 17.22  Breeding true connection between B and w. B and b are obvious partners. In the same way L could represent the allele Genotype and phenotype for long fur and l the allele for short fur. The two kinds of black rabbit BB and Bb are said to Breeding true have the same phenotype. This is because their coat colours look exactly the same. However, because they A white rabbit must have both the recessive alleles have different allele pairs for coat colour they are said to b and b. If it had B and b, the dominant allele for have different genotypes, i.e. different combinations of black (B) would override the allele for white (b) and alleles. One genotype is BB and the other is Bb. produce a black rabbit. A black rabbit, on the other hand, could be either BB or Bb and, by just looking You and your brother might both be brown-eyed at the rabbit, you could not tell the difference. When phenotypes but your genotype could be BB and his a male black rabbit BB produces sperm, each one of could be Bb. You would be homozygous dominant for the pair of chromosomes carrying the B alleles will brown eyes; he would be heterozygous for eye colour. end up in different sperm cells. Since the alleles are the same, all the sperm will have the B allele for black The three to one ratio fur (Figure 17.22(a)). The result of a mating between a true-breeding A black rabbit BB is called a true-breeding black and is (homozygous) black mouse (BB) and a true-breeding said to be homozygous for black coat colour (‘homo-’ (homozygous) brown mouse (bb) is shown in means ‘the same’). If this rabbit mates with another Figure 17.23(a). The illustration is greatly simplified black (BB) rabbit, all the babies will be black because all because it shows only one pair of the 20 pairs of will receive a dominant allele for black fur. When all the mouse chromosomes and only one pair of alleles on offspring have the same characteristic as the parents, this the chromosomes. is called ‘breeding true’ for this characteristic. Because black is dominant to brown, all the When a Bb black rabbit produces gametes by offspring from this mating will be black phenotypes, meiosis, the chromosomes with the B allele and because they all receive the dominant allele for black the chromosomes with the b allele will end up fur from the father. Their genotypes, however, will be in different gametes. So 50% of the sperm cells Bb because they all receive the recessive b allele from will carry B alleles and 50% will carry b alleles the mother. They are heterozygous for coat colour. (Figure 17.22(b)). Similarly, in the female, 50% of the The offspring resulting from this first mating are ova will have a B allele and 50% will have a b allele. If called the F1 generation. a b sperm fertilises a b ovum, the offspring, with two b alleles (bb), will be white. The black Bb rabbits are 261

17 Inheritance Figure 17.23(b) shows what happens when these other parent. The parent’s genotype is written to heterozygous, F1 black mice are mated together to the left. The genotypes of the offspring can then be produce what is called the F2 generation. Each sperm predicted by completing the four boxes, as shown. or ovum produced by meiosis can contain only one of In this example, two heterozygous tall organisms the alleles for coat colour, either B or b. So there are (Tt) are the parents. The genotypes of the offspring two kinds of sperm cell, one kind with the B allele and are TT, Tt, Tt and tt. We know that the allele T is one kind with the b allele. There are also two kinds dominant because the parents are tall, although they of ovum, with either B or b alleles. When fertilisation carry both tall and dwarf alleles. So, the phenotypes occurs, there is no way of telling whether a b or a B of the offspring will be three tall to one dwarf. sperm will fertilise a B or a b ovum, so we have to look at all the possible combinations as follows: BB bb l A b sperm fertilises a B ovum. Result: bB zygote. homozygous black male homozygous brown female l A b sperm fertilises a b ovum. Result: bb zygote. l A B sperm fertilises a B ovum. Result: BB zygote. cell BB bb cell l A B sperm fertilises a b ovum. Result: Bb zygote. in in There is no difference between bB and Bb, so there testis ovary are three possible genotypes in the offspring – BB, Bb and bb. There are only two phenotypes – black BB bb (BB or Bb) and brown (bb). So, according to the laws of chance, we would expect three black baby mice sperms ova and one brown. Mice usually have more than four offspring and what we really expect is that the ratio b (proportion) of black to brown will be close to 3:1. B If the mouse had 13 babies, you might expect nine (all possible black and four brown, or eight black and five brown. combinations Even if she had 16 babies you would not expect to find are the same) exactly 12 black and four brown because whether a B or b sperm fertilises a B or b ovum is a matter of chance. If Bb you spun ten coins, you would not expect to get exactly five heads and five tails. You would not be surprised at Bb Bb Bb Bb six heads and four tails or even seven heads and three tails. In the same way, we would not be surprised at 14 (a) all the F1 generation are heterozygous black black and two brown mice in a litter of 16. Figure 17.23  Inheritance of coat colour in mice To decide whether there really is a 3:1 ratio, we need a lot of results. These may come either from breeding the same pair of mice together for a year or so to produce many litters, or from mating 20 black and 20 brown mice, crossing the offspring and adding up the number of black and brown babies in the F2 families (see also Figure 17.24). When working out the results of a genetic cross, it is useful to display the outcomes in a ‘Punnett square’ (Figure 17.25). This a box divided into four compartments. The two boxes along the top are labelled with the genotypes of the gametes of one parent. The genotypes are circled to show they are gametes. The parent’s genotype is written above the gametes. The boxes down the left-hand side are labelled with the genotypes of the gametes of the 262

Monohybrid inheritance PARENTS Bb Bb MEIOSIS heterozygous black male heterozygous black female cell Bb Bb cell in in testis ovary GAMETES sperms (two B b B b ova (two possibilities) possibilities) FERTILISATION B b B b b (four possible B B b combinations) POSSIBLE BB bB Bb bb ZYGOTES OFFSPRING BB bB Bb bb (b) the probable ratio of coat colours in the F2 generation is 3 black : 1 brown Figure 17.23  Inheritance of coat colour in mice (continued) Tt Tt T TT Tt Tt Tt tt t Figure 17.24  F2 hybrids in maize. In the two left-hand cobs, the grain Figure 17.25  Using a Punnett square to predict the outcomes of a colour phenotypes appear in a 3:1 ratio (try counting single rows in the lighter genetic cross cob). What was the colour of the parental grains for each of these cobs? 263

17 Inheritance The recessive test-cross (back-cross) Since the alleles for groups A and B are dominant to that for group O, a group A person could have the A black mouse could have either the BB or the Bb genotype IAIA or IAIo. Similarly a group B person genotype. One way to find out which is to cross the could be IBIB or IBIo. There are no alternative black mouse with a known homozygous recessive genotypes for groups AB and O. mouse, bb. The bb mouse will produce gametes with only the recessive b allele. A black homozygote, Inheritance of blood group O BB, will produce only B gametes. Thus, if the black Blood group O can be inherited, even though mouse is BB, all the offspring from the cross will be neither parent shows this phenotype. black heterozygotes, Bb. Two parents have the groups A and B. The father Half the gametes from a black Bb mouse would is IAIo and the mother is IBIo (Figure 17.26). carry the B allele and half would have the b allele. So, if the black mouse is Bb, half of the offspring from Phenotypes of parents blood group A blood group B the cross will, on average, be brown homozygotes, Genotypes of parents IAIo × IBIo bb, and half will be black heterozygotes, Bb. Gametes IA Io × IB Io The term ‘back-cross’ refers to the fact that, in Punnett square effect, the black, mystery mouse is being crossed IAIo with the same genotype as its brown grandparent, IA Io the bb mouse in Figure 17.23(a). Mouse ethics and speed of reproduction make the use of the actual grandparent quite feasible! Co-dominance and incomplete dominance Co-dominance IB IAIB IBIo If both genes of an allelomorphic pair produce their IBIo IAIo IoIo effects in an individual (i.e. neither allele is dominant to the other) the alleles are said to be co-dominant. Io The inheritance of the human ABO blood groups F1 genotypes IAIo IBIo IAIB IoIo provides an example of co-dominance. In the ABO system, there are four phenotypic blood groups, A, F1 phenotypes A B AB O B, AB and O. The alleles for groups A and B are co-dominant. If a person inherits alleles for group Ratio 1: 1: 1: 1 A and group B, his or her red cells will carry both antigen A and antigen B. Figure 17.26  Inheritance of blood group O However, the alleles for groups A and B are both Some plants show co-dominance with regard to completely dominant to the allele for group O. petal colour. For example, with the gene for flower (Group O people have neither A nor B antigens on colour in the geranium, the alleles are CR (red) and their red cells.) CW (white). The capital letter ‘C’ has been chosen to represent colour. Pure breeding (homozygous) Table 17.2 shows the genotypes and phenotypes flowers may be red (CRCR) or white (CWCW). If for the ABO blood groups. (Note that the allele these are cross-pollinated, all the first filial (F1) for group O is sometimes represented as Io and generation will be heterozygous (CRCW) and they sometimes as i.) are pink because both alleles have an effect on the phenotype. Table 17.2  The ABO blood groups Self-pollinating the pink (F1) plants results in an Genotype Blood group (phenotype) unusual ratio in the next (F2) generation of 1 red : 2 IAIA or IAIo A pink : 1 white. IBIB or IBIo B IAIB AB IoIo O 264

Monohybrid inheritance Incomplete dominance One example of this is a form of colour blindness This term is sometimes taken to mean the same (Figure 17.27). In the following case, the mother as ‘co-dominance’ but, strictly, it applies to a is a carrier of colour blindness (XCXc). This means case where the effect of the recessive allele is not she shows no symptoms of colour blindness, but the completely masked by the dominant allele. recessive allele causing colour blindness is present on one of her X chromosomes. The father has normal An example occurs with sickle-cell anaemia (see colour vision (XCY). ‘Variation’ in Chapter 18). If a person inherits both recessive alleles (HbSHbS) for sickle-cell Phenotypes of parents mother: father: haemoglobin, then he or she will exhibit signs of normal vision normal vision the disease, i.e. distortion of the red cells leading to severe bouts of anaemia. Genotypes of parents XCXc × XCY A heterozygote (HbAHbS), however, will have Gametes XC Xc × XC Y a condition called ‘sickle-cell trait’. Although there Punnett square may be mild symptoms of anaemia the condition XCXc is not serious or life-threatening. In this case, the XC Xc normal haemoglobin allele (HbA) is not completely dominant over the recessive (HbS) allele. XCY XC XCXC XCXc Y XCY XcY Sex linkage Key definitions A sex-linked characteristic is one in which the gene responsible is located on a sex chromosome, which makes it more common in one sex than the other. The sex chromosomes, X and Y, carry genes that F1 genotypes XCXC XCXc XCY XcY control sexual development. In addition they carry genes that control other characteristics. These tend to F1 phenotypes 2 females with normal vision; 2 males, be on the X chromosome, which has longer arms to one with normal vision, the chromatids. Even if the allele is recessive, because one with colour blindness there is no corresponding allele on the Y chromosome, it is bound to be expressed in a male (XY). There is less Figure 17.27  Inheritance of colour blindness chance of a recessive allele being expressed in a female (XX) because the other X chromosome may carry the If the gene responsible for a particular condition is dominant form of the allele. present only on the Y chromosome, only males can suffer from the condition because females do not possess the Y chromosome. ●● Extension work characteristics. This he did by using varieties of the pea plant, Pisum sativum, which he grew in the Ideas about heredity: Gregor Mendel monastery garden. He chose pea plants because they (1822–84) were self-pollinating (Chapter 16). Pollen from the anthers reached the stigma of the same flower even Mendel was an Augustinian monk from the town before the flower bud opened. of Brünn (now Brno) in Czechoslovakia (now the Czech Republic). He studied maths and science at the Mendel selected varieties of pea plant that bore University of Vienna in order to teach at a local school. distinctive and contrasting characteristics, such as green seeds vs yellow seeds, dwarf vs tall, round He was the first scientist to make a systematic seeds vs wrinkled (Figure 17.28). He used only study of patterns of inheritance involving single plants that bred true. 265

17 Inheritance fruit shape seed shape l round vs wrinkled seeds 5474:1850 = 2.96:1 l yellow vs green seeds 6022:2001 = 3.01:1 round wrinkled l green vs yellow pods   428:152 = 2.82:1 inflated constricted cotyledon colour Two-thirds of the dominant tall F2 plants did not fruit (pod) colour green yellow breed true when self-pollinated but produced the 3:1 ratio of tall : dwarf. They were therefore similar seed coat (testa) colour to the plants of the F1 generation. green yellow grey white It is not clear whether Mendel speculated on how the characteristics were represented in the gametes Figure 17.28  Some of the characteristics investigated by Mendel or how they achieved their effects. At one point he wrote of ‘the differentiating elements of the egg He then crossed pairs of the contrasting varieties. and pollen cells’, but it is questionable whether he To do this he had to open the flower buds, remove envisaged actual structures being responsible. the stamens and use them to dust pollen on the stigmas of the contrasting variety. The offspring of Similarly, when Mendel wrote ‘exactly similar this cross he called the ‘first filial’ generation, or F1. factors must be at work’, he meant that there must be similar processes taking place. He does not use The first thing he noticed was that all the offspring the term ‘factor’ to imply particles or any entities of the F1 cross showed the characteristic of only one that control heritable characteristics. of the parents. For example, tall plants crossed with dwarf plants produced only tall plants in the first His symbols A, Ab and b seem to be shorthand generation. for the types of plants he studied: A = true-breeding dominant, b = true-breeding recessive and Ab = the Next he allowed the plants of the F1 generation non-true-breeding ‘hybrid’. The letters represented to self-pollinate and so produce a second filial the visible characteristics, whereas today they generation, or F2. Surprisingly, the dwarf characteristic represent the alleles responsible for producing the that had, seemingly, disappeared in the F1 reappeared characteristic. For example, Mendel never refers to in the F2. This characteristic had not, in fact, been AA or bb so he probably did not appreciate that lost but merely concealed or suppressed in the F1 each characteristic is represented twice in the somatic to re-emerge in the F2. Mendel called the repressed cells but only once in the gametes. feature ‘recessive’ and the expressed feature ‘dominant’. When Mendel crossed plants, each carrying two contrasting characteristics, he found that Also, it must be noted, the plants were all either the characteristics turned up in the offspring tall or dwarf; there were no intermediates, as might independently of each other. For example, in a cross be expected if the characteristics blended. between a tall plant with green seeds and a dwarf plant with yellow seeds, some of the offspring were tall with Mendel noticed that pollen from tall plants, yellow seeds and some dwarf with green seeds. transferred to the stigmas of short plants, produced the same result as transferring pollen from short So, Mendel’s work was descriptive and plants to the stigmas of tall plants. This meant that mathematical rather than explanatory. He showed male and female gametes contributed equally to the that certain characteristics were inherited in a observed characteristic. predictable way, that the gametes were the vehicles, that these characteristics did not blend but retained When Mendel counted the number of contrasting their identity and could be inherited independently offspring in the F2, he found that they occurred in of each other. He also recognised dominant and the ratio of three dominant to one recessive. For recessive characteristics and, by ‘hybridisation’, that example, of 1064 F2 plants from the tall × dwarf in the presence of the dominant characteristic the cross, 787 were tall and 277 dwarf, a ratio of 2.84:1. recessive characteristic, though not expressed, did This F2 ratio occurred in all Mendel’s crosses, for not ‘disappear’. example: Mendel published his results in 1866 in ‘Transactions of the Brünn Natural History Society’, which, understandably, did not have a 266

Monohybrid inheritance wide circulation. Only when Mendel’s work was ● The first ‘law’ (the law of segregation) is expressed rediscovered in 1900 was the importance and as ‘of a pair of contrasted characters only one can significance of his findings appreciated. be represented in the gamete’. Mendel’s observations are sometimes summarised ● The second ‘law’ (the law of independent in the form of ‘Mendel’s laws’, but Mendel did not assortment) is given as ‘each of a pair of formulate any laws and these are the product of contrasting characters may be combined with modern knowledge of genetics. either of another pair’. Questions 13 How many chromosomes would there be in the nucleus Core of: 1 A married couple has four girl children but no boys. This a a human muscle cell b a mouse kidney cell does not mean that the husband produces only X sperms. c a human skin cell that has just been produced by mitosis Explain why not. d a kangaroo sperm cell? 2 Which sex chromosome determines the sex of a baby? 14 What is the diploid number in humans? Explain your answer. 15 Suggest why sperm could be described as male sperm and 3 Some plants occur in one of two sizes, tall or dwarf. This characteristic is controlled by one pair of genes. Tallness female sperm. is dominant to shortness. Choose suitable letters for the 16 a What are gametes? gene pair. 4 Why are there two types of gene controlling one b What are the male and female gametes of characteristic? Do the two types affect the characteristic in i plants and the same way as each other? ii animals called, and where are they produced? 5 The allele for red hair is recessive to the allele for black hair. What colour hair will a person have if he inherits an c What happens at fertilisation? allele for red hair from his mother and an allele for black d What is a zygote and what does it develop into? hair from his father? 17 How many chromatids will there be in the nucleus of a 6 a Read Question 5 again. Choose letters for the alleles human cell just before cell division? for red hair and black hair and write down the allele 18 Why can chromosomes not be seen when a cell is not combination for having red hair. b Would you expect a red-haired couple to breed true? dividing? c Could a black-haired couple have a red-haired baby? 19 In which human tissues would you expect mitosis to be 7 Use the words ‘homozygous’, ‘heterozygous’, ‘dominant’ and ‘recessive’ (where suitable) to describe the following going on, in: allele combinations: Aa, AA, aa. a a 5-year-old child 8 A plant has two varieties, one with red petals and one b an adult? with white petals. When these two varieties are cross- 20 What is the haploid number for: pollinated, all the offspring have red petals. Which allele a a human is dominant? Choose suitable letters to represent the two b a fruit fly? alleles. 21 Which of the following cells would be haploid and which 9 Look at Figure 17.23(a). Why is there no possibility of getting a BB or a bb combination in the offspring? diploid: white blood cell, male cell in pollen grain, guard 10 In Figure 17.23(b) what proportion of the F2 black mice are true-breeding? cell, root hair, ovum, sperm, skin cell, egg cell in ovule? 11 Two black guinea-pigs are mated together on several 22 Where in the body of the following organisms would you occasions and their offspring are invariably black. However, when their black offspring are mated with white guinea- expect meiosis to be taking place? pigs, half of the matings result in all black litters and a a human male the other half produce litters containing equal numbers b a human female of black and white babies. From these results, deduce c a flowering plant the genotypes of the parents and explain the results of 23 How many chromosomes would be present in: the various matings, assuming that colour in this case is a a mouse sperm cell determined by a single pair of alleles. b a mouse ovum? 24 Why are organisms that are produced by asexual Extended 12 How many bases will there be in an mRNA molecule reproduction identical to each other? 25 Two black rabbits thought to be homozygous for coat coding for haemoglobin? colour were mated and produced a litter that contained all black babies. The F2, however, resulted in some white babies, which meant that one of the grandparents was heterozygous for coat colour. How would you find out which grandparent was heterozygous? 26 What combinations of blood groups can result in a child being born with blood group O? Use Punnett squares to show your reasoning. 267

17 InherItanCe 27 A woman of blood group A claims that a man of blood a Is this an example of co-dominance or incomplete group AB is the father of her child. A blood test reveals dominance? that the child’s blood group is O. a Is it possible that the woman’s claim is correct? b What coat colours would you expect among the b Could the father have been a group B man? offspring of a mating between two roan cattle? Explain your reasoning. 29 Predict the ratio of children with colour blindness resulting 28 A red cow has a pair of alleles for red hairs. A white bull has from a mother who is a carrier for colour blindness having a pair of alleles for white hairs. If a red cow and a white bull children with a father who is colour blind. are mated, the offspring are all ‘roan’, i.e. they have red and white hairs equally distributed over their body. Checklist • A diploid nucleus is a nucleus containing two sets of chromosomes (e.g. in body cells). After studying Chapter 17 you should know and understand the following: • In a diploid cell, there is a pair of each type of chromosome; in a human diploid cell there are 23 pairs. • Inheritance is the transmission of genetic information from generation to generation. Mitosis Chromosomes, genes and proteins • Mitosis is nuclear division giving rise to genetically identical cells. • A chromosome is a thread of DNA, made up of a string of genes. • Mitosis is important in growth, repair of damaged tissues, replacement of cells and in asexual reproduction. • A gene is a length of DNA that codes for a protein. • An allele is a version of a gene. • Before mitosis, the exact duplication of chromosomes • Chromosomes are found as thread-like structures in the occurs. nuclei of all cells. • Each species of plant or animal has a fixed number of • Chromosomes are in pairs; one of each pair comes from the chromosomes in its cells. male and one from the female parent. • When cells divide by mitosis, the chromosomes and genes • Sex, in mammals, is determined by the X and Y are copied exactly and each new cell gets a full set. chromosomes. Males are XY; females are XX. • Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for • The DNA molecule is coiled along the length of the specific purposes. chromosome. Meiosis • A DNA molecule is made up of a double chain of nucleotides in the form of a helix. • Meiosis is reduction division in which the chromosome number is halved from diploid to haploid resulting in • The nucleotide bases in the helix pair up A–T and C–G. genetically different cells. • Triplets of bases control production of the specific amino • Gametes are the result of meiosis. acids that make up a protein. • At meiosis, only one chromosome of each pair goes into • Genes consist of specific lengths of DNA. • Most genes control the type of enzyme that a cell will make. the gamete. • When proteins are made: • Meiosis produces variation by forming new combinations – the DNA with the genetic code for the protein remains of maternal and paternal chromosomes. in the nucleus Monohybrid inheritance – mRNA molecules carry a copy of the genetic code to the cytoplasm • The genotype of an organism is its genetic make-up. • The phenotype of an organism is its features. – the mRNA passes through ribosomes in the cytoplasm • Homozygous means having two identical alleles of a and the ribosome puts together amino acids to form protein molecules. particular gene. Two identical homozygous individuals that breed together will be pure-breeding. • The specific order of amino acids is decided by the • Heterozygous means having two different alleles of a sequence of bases in the mRNA. particular gene. A heterozygous individual will therefore not be pure-breeding. • All body cells in an organism contain the same genes, but • A dominant allele is one that is expressed if it is present. many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs. • A haploid nucleus is a nucleus containing a single set of unpaired chromosomes (e.g. in sperm and egg cells). 268

Monohybrid inheritance • A recessive allele is one that is only expressed when there is • The inheritance of ABO blood groups is an example of no dominant allele of the gene present. co-dominance. • Genetic diagrams are used to predict the results of • The phenotypes are A, B, AB and O blood groups. monohybrid crosses and calculate phenotypic ratios. • The genotypes are IA, IB and Io. • A sex-linked characteristic is a characteristic in which the • Punnett squares can be used in crosses to work out and show the possible different genotypes. gene responsible is located on a sex chromosome. This makes it more common in one sex than in the other. • A test-cross is used to identify an unknown genotype, for • Colour blindness is an example of sex linkage. instance to find out if it is pure breeding or heterozygous. • Genetic diagrams can be used to predict the results of monohybrid crosses involving co-dominance and sex • In some cases, neither one of a pair of alleles is linkage. fully dominant over the other. This is called co-dominance. 269

18 Variation and selection Variation Define adaptive feature, fitness Define variation Adaptive features of hydrophytes and xerophytes Discontinuous and continuous variation Define mutation Selection Causes of mutations Natural selection Artificial selection Causes of discontinuous and continuous variation Selective breeding Define gene mutation Sickle-cell anaemia Define the process of adaptation Down’s syndrome Evolution Mutations in bacteria Development of strains of resistant bacteria Use of selective breeding Adaptive features Compare natural and artificial selection Define adaptive feature Describe adaptive features of organisms ●● Variation genes for producing pigment will only go brown if he or she exposes themselves to sunlight. So the tan is a Key definition result of both inherited and acquired characteristics. Variation is the differences between individuals of the same north side, upper branches south side, upper branches species. The term ‘variation’ refers to observable differences north side, lower branches south side, lower branches within a species. All domestic cats belong to the same species, i.e. they can all interbreed, but Figure 18.1 Acquired characteristics. These apples have all been picked there are many variations of size, coat colour, eye from different parts of the same tree. All the apples have similar genotypes, colour, fur length, etc. Those variations that can be so the differences in size must have been caused by environmental effects. inherited are determined by genes. They are genetic variations. Phenotypic variations may be brought Discontinuous variation about by genes, but can also be caused by the environment, or a combination of both genes and In discontinuous variation, the variations take the the environment. form of distinct, alternative phenotypes with no intermediates (Figures 18.2 and 18.4). The mice in So, there are variations that are not heritable, but Figure 17.23 are either black or brown; there are no determined by factors in the environment. A kitten intermediates. You are either male or female. Apart that gets insufficient food will not grow to the same from a small number of abnormalities, sex is inherited size as its litter mates. A cat with a skin disease may in a discontinuous way. Some people can roll their have bald patches in its coat. These conditions are not tongue into a tube. Others are unable to do it. They heritable. They are caused by environmental effects. Similarly, a fair-skinned person may be able to change the colour of his or her skin by exposing it to the Sun, so getting a tan. The tan is an acquired characteristic. You cannot inherit a suntan. Black skin, on the other hand, is an inherited characteristic. Many features in plants and animals are a mixture of acquired and inherited characteristics (Figure 18.1). For example, some fair-skinned people never go brown in the Sun, they only become sunburned. They have not inherited the genes for producing the extra brown pigment in their skin. A fair-skinned person with the 270

Variation are known as non-tongue rollers. Again, there are no There are many characteristics that are difficult intermediates (Figure 18.2). to classify as either wholly continuous or discontinuous variations. Human eye colour has 20 already been mentioned. People can be classified 18 roughly as having blue eyes or brown eyes, but there are also categories described as grey, hazel 16 or green. It is likely that there are a small number of genes for eye colour and a dominant gene for frequency 14 brown eyes, which overrides all the others when 12 it is present. Similarly, red hair is a discontinuous 10 variation but it is masked by genes for other colours and there is a continuous range of hair colour from 8 blond to black. 6 Mutations 4 Key definition A mutation is a spontaneous genetic change. Mutation is the 2 non-rollers 0 rollers way new alleles are formed. Figure 18.2   Discontinuous variation. Tongue rollers and non-rollers Many of the cat coat variations mentioned overleaf in a class may have arisen, in the first place, as mutations in a wild stock of cats. A recent variant produced by a Discontinuous variation cannot usually be altered by mutation is the ‘rex’ variety, in which the coat has the environment. You cannot change your eye colour curly hairs. by altering your diet. A genetic dwarf cannot grow taller by eating more food. You cannot learn how to Many of our high-yielding crop plants have roll your tongue. arisen as a result of mutations in which the whole chromosome set has been doubled. Continuous variation Exposure to mutagens, namely certain chemicals An example of continuous variation is height. There and radiation, is known to increase the rate of are no distinct categories of height; people are not mutation. Some of the substances in tobacco either tall or short. There are all possible intermediates smoke, such as tar, are mutagens, which can cause between very short and very tall (Figure 18.3). cancer. thousands of men 14 Ionising radiation from X-rays and radioactive compounds, and ultraviolet radiation from sunlight, 12 can both increase the mutation rate. It is uncertain whether there is a minimum dose of radiation 10 below which there is negligible risk. It is possible that repeated exposure to low doses of radiation 8 is as harmful as one exposure to a high dose. It has become clear in recent years that, in light- 6 skinned people, unprotected exposure to ultraviolet radiation from the Sun can cause a form of skin 4 cancer. 2 Generally speaking, however, exposure to natural and medical sources of radiation carries less risk than 0 smoking cigarettes or driving a car, but it is sensible 145 150 155 160 165 170 175 180 185 190 to keep exposure to a minimum. height/cm Figure 18.3  Continuous variation. Heights of 90 000 army recruits. The apparent ‘steps’ in the distribution are the result of arbitrarily chosen categories, differing in height by 1 cm. But heights do not differ by exactly 1 cm. If measurements could be made accurately to the nearest millimetre there would be a smooth curve like the one shown in colour. 271

18 Variation and selection Genetic variation may be the result of new intelligence, are always the result of interaction combinations of genes in the zygote, or mutations. between the genotype and the environment. Discontinuous variation New combinations of genes Discontinuous variation is under the control of a If a grey cat with long fur is mated with a black cat single pair of alleles or a small number of genes. with short fur, the kittens will all be black with short An example is human blood groups. These were fur. If these offspring are mated together, in due discussed in Chapter 17. course the litters may include four varieties: black– short, black–long, grey–short and grey–long. Two of A person is one of four blood groups: A, B, AB or these are different from either of the parents. O. There are no groups in between. Mutation 46 42 Key definition A gene mutation is a change in the base sequence in DNA. percentage in population of Britain A mutation may occur in a gene or a chromosome. 9 In a gene mutation it may be that one or more 3 genes are not replicated correctly. A chromosome mutation may result from damage to or loss of part AB A O B of a chromosome during mitosis or meiosis, or even blood group the gain of an extra chromosome, as in Down’s syndrome (see page 273). Figure 18.4  Discontinuous variation. Frequencies of ABO blood groups in Britain. The figures could not be adjusted to fit a An abrupt change in a gene or chromosome is smooth curve because there are no intermediates. likely to result in a defective enzyme and will usually disrupt the complex reactions in the cells. Most Continuous variation mutations, therefore, are harmful to the organism. Continuous variation is influenced by a combination Surprisingly, only about 3% of human DNA of both genetic and environmental factors. consists of genes. The rest consists of repeated Continuously variable characteristics are usually sequences of nucleotides that do not code for controlled by several pairs of alleles. There might be proteins. This is sometimes called ‘junk DNA’, five pairs of alleles for height – (Hh), (Tt), (Ll), (Ee) but that term only means that we do not know its and (Gg) – each dominant allele adding 4 cm to your function. If mutations occur in these non-coding height. If you inherited all ten dominant genes (HH, sequences they are unlikely to have any effect on the TT, etc.) you could be 40 cm taller than a person organism and are, therefore, described as ‘neutral’. who inherited all ten recessive genes (hh, tt, etc.). Rarely, a gene or chromosome mutation produces The actual number of genes that control height, a beneficial effect and this may contribute to the intelligence, and even the colour of hair and skin, is success of the organism (see ‘Selection’ later in this not known. chapter). Continuously variable characteristics are greatly If a mutation occurs in a gamete, it will affect all influenced by the environment. A person may inherit the cells of the individual that develops from the genes for tallness and yet not get enough food to zygote. Thus the whole organism will be affected. If grow tall. A plant may have the genes for large fruits the mutation occurs in a somatic cell (body cell), it but not get enough water, minerals or sunlight will affect only those cells produced, by mitosis, from to produce large fruits. Continuous variations in the affected cell. human populations, such as height, physique and Thus, a mutation in a gamete may result in a genetic disorder, e.g. haemophilia or cystic fibrosis. Mutations in somatic cells may give rise to cancers by promoting uncontrolled cell division in the 272

Variation affected tissue. For example, skin cancer results HbAHbS HbAHbS from uncontrolled cell division in the basal layer of the skin. HbAHbA HbAHbS HbAHbS HbSHbS A mutation may be as small as the substitution reduced survival; positive selection reduced survival; of one organic base for another in the DNA selected against due to malaria selected against molecule, or as large as the breakage, loss or gain by malaria resistance by illness of a chromosome. S ickle-cell anaemia Figure 18.6  Selection in sickle-cell disease This condition has already been mentioned in Chapter 17. A person with sickle-cell disease With sickle-cell anaemia, the defective haemoglobin has inherited both recessive alleles (HbSHbS) molecule differs from normal haemoglobin by only one for defective haemoglobin. The distortion and amino acid (represented by a sequence of three bases), destruction of the red cells, which occurs in low i.e. valine replaces glutamic acid. This could be the oxygen concentrations, leads to bouts of severe result of faulty replication at meiosis. When the relevant anaemia (Figure 18.5). In many African countries, parental chromosome replicated at gamete formation, sufferers have a reduced chance of reaching the DNA could have produced the triplet –CAT– reproductive age and having a family. There is thus (which specifies valine) instead of –CTT– (which a selection pressure, which tends to remove the specifies glutamic acid). In this case, a change of just homozygous recessives from the population. In such one base (from A to T) makes a significant difference a case, you might expect the harmful HbS allele to the characteristics of the protein (haemoglobin). to be selected out of the population altogether. However, the heterozygotes (HbAHbS) have Down’s syndrome virtually no symptoms of anaemia but do have the Down’s syndrome is a form of mental and physical advantage that they are more resistant to malaria disability, which results from a chromosome mutation. than the homozygotes HbAHbA. It appears that the During the process of meiosis which produces an malaria parasite is unable to invade and reproduce in ovum, one of the chromosomes (chromosome 21) the sickle cells. fails to separate from its homologous partner, a process known as non-disjunction. As a result, the The selection pressure of malaria, therefore, ovum carries 24 chromosomes instead of 23, and favours the heterozygotes over the homozygotes the resulting zygote has 47 instead of the normal 46 and the potentially harmful HbS allele is kept in the chromosomes. The risk of having a baby with Down’s population (Figure 18.6). syndrome increases as the mother gets older. When Africans migrate to countries where malaria Mutations in bacteria does not occur, the selective advantage of the HbS Mutations in bacteria often produce resistance to allele is lost and the frequency of this allele in the drugs. Bacterial cells reproduce very rapidly, perhaps population diminishes. as often as once every 20 minutes. Thus a mutation, even if it occurs only rarely, is likely to appear in a Figure 18.5  Sickle-cell anaemia (×800). At low oxygen concentration large population of bacteria. If a population of the red cells become distorted. bacteria containing one or two drug-resistant mutants is subjected to that particular drug, the non-resistant bacteria will be killed but the drug- resistant mutants survive (see Figure 15.1). Mutant genes are inherited in the same way as normal genes, so when the surviving mutant bacteria reproduce, all their offspring will be resistant to the drug. Mutations are comparatively rare events; perhaps only one in every 100 000 replications results in a mutation. Nevertheless they do occur naturally all the time. 273

18 Variation and selection ●● Adaptive features ball and hinge five groups of bones, socket joint joint Key definition (shoulder) (elbow) each arranged in a ‘chain’1 2 An adaptive feature is an inherited feature that helps an (hand and fingers) organism to survive and reproduce in its environment. 3 Adaptation one bone two bones group of 4 (humerus) (radius and ulna) 5 When biologists say that a plant or animal is adapted to its habitat they usually mean that, in the course of small bones (wrist) evolution, changes have occurred in the organism, which make it more successful in exploiting its (a) pattern of bones in human forelimb habitat, e.g. animals finding and digesting food, selecting nest sites or hiding places, or plants humerus radius wrist exploiting limited mineral resources or tolerating 1 salinity or drought. It is tempting to assume that because we find a plant or animal in a particular 2 habitat it must be adapted to its habitat. There is some logic in this; if an organism was not adapted 3 to its habitat, presumably it would be eliminated by natural selection. However, it is best to look for ulna 54 positive evidence of adaptation. (b) whale Sometimes, just by looking at an organism and comparing it with related species, it is possible to make Figure 18.8  Skeletons of the forelimbs of human and whale reasoned guesses about adaptation. For example, there seems little doubt that the long, hair-fringed hind legs The camel of a water beetle are adaptations to locomotion in Camels are adapted to survive in a hot, dry and water when compared with the corresponding legs of sandy environment. Adaptive physical features are a land-living relative (Figure 18.7). closable nostrils and long eyelashes, which help keep out wind-blown sand (Figure 18.9). Their feet long legs are broad and splay out under pressure, so reducing with fringe the tendency to sink into the sand. Thick fur of bristles insulates the body against heat gain in the intense sunlight. (a) water beetle (b) ground beetle Physiologically, a camel is able to survive without Figure 18.7  Adaptation to locomotion in water and on land water for 6–8 days. Its stomach has a large water- holding capacity, though it drinks to replace water Similarly, in Figure 18.8 it seems reasonable to lost by evaporation rather than in anticipation of suppose that, compared with the generalised water deprivation. mammalian limb, the forelimbs of whales are adapted for locomotion in water. The body temperature of a ‘thirsty’ camel rises to as much as 40 °C during the day and falls to about By studying animals which live in extreme habitats, 35 °C at night. The elevated daytime temperature it is possible to suggest ways in which they might be reduces the heat gradient between the body and the adapted to these habitats especially if the observations surroundings, so less heat is absorbed. A camel is able are supported by physiological evidence. to tolerate water loss equivalent to 25% of its body weight, compared with humans for whom a 12% loss may be fatal. The blood volume and concentration are maintained by withdrawing water from the body tissues. The nasal passages are lined with mucus. During exhalation, the dry mucus absorbs water vapour. During inhalation the now moist mucus adds water vapour to the inhaled air. In this way, water is conserved. The role of the camel’s humps in water conservation is more complex. The humps contain fat and are therefore an important reserve of energy- giving food. However, when the fat is metabolised during respiration, carbon dioxide and water 274

Adaptive features Figure 18.9  Protection against wind-blown sand. The nostrils are slit- Figure 18.11  The heavy coat and small ears also help the polar bear to like and can be closed. The long eyelashes protect the eyes reduce heat losses. (metabolic water) are produced. The water enters The principal thermal insulation comes from a 10 cm the blood circulation and would normally be lost by layer of fat (blubber) beneath the skin. The thermal evaporation from the lungs, but the water-conserving conductivity of fat is little different from any other nasal mucus will trap at least a proportion of it. tissue but it has a limited blood supply. This means The polar bear that very little warm blood circulates close to the Polar bears live in the Arctic, spending much skin surface. of their time on snow and ice. Several physical features contribute to their adaptation to this cold The hollow hairs of the white fur are thought to environment. transmit the Sun’s heat to the black skin below. Black is an efficient colour for absorbing heat. The white It is a very large bear (Figure 18.10), which colour is also probably an effective camouflage when means that the ratio of its surface area to its volume hunting its prey, mainly seals. is relatively small. The relatively small surface area means that the polar bear loses proportionately less A specific adaptation to walking on snow and ice heat than its more southerly relatives. Also its ears is the heat-exchange arrangement in the limbs. The are small, another feature that reduces heat loss arteries supplying the feet run very close to the veins (Figure 18.11). returning blood to the heart. Heat from the arteries is transferred to the veins before the blood reaches It has a thick coat with long, loosely packed coarse the feet (Figure 18.12). So, little heat is lost from hairs (guard hairs) and a denser layer of shorter woolly the feet but their temperature is maintained above hairs forming an insulating layer. The long hairs are freezing point, preventing frost-bite. oily and water-repellent and enable the bear to shake off water when it emerges from a spell of swimming. Polar bears breed in winter when temperatures fall well below zero. However, the pregnant female excavates a den in the snow in which to give birth and rear her two cubs. In this way the cubs are protected from the extreme cold. The female remains in the den for about 140 days, suckling her young on the rich milk, which is formed from her fat reserves. Figure 18.10  The polar bear and the sun bear (from SE Asia). The Venus flytrap smaller surface area/volume ratio in the polar bear helps conserve heat. Many plants show adaptions as well as animals. Insectivorous plants such as the Venus flytrap (Figure 18.13) live in habitats where there is often a shortage of nitrates for growth. They have developed pairs of leaves with tooth-like edges. The leaves have 275

18 Variation and selection Other adaptations Adaptive features of the long-eared bat and the hare key are illustrated in Figures 18.14 and 18.15. warm blood cool blood heat is transferred Figure 18.14 Long-eared bat. The bat gives out high-pitched sounds, from the artery to which are reflected back from its prey and from obstacles, to its ears and the vein sensitive patches on its face. By timing these echoes the bat can judge the blood supply its distance from the obstacle or prey. This allows it to fly and feed in the to the foot is dark. Its body is covered in fur for insulation. Its forearms are covered by a maintained but membrane of skin to form a wing. The fingers are very long to stretch out heat loss is the membrane to increase the surface area of the wing. minimised Figure 18.12 The heat-exchange mechanism in the polar bear’s limb sensitive hairs on their surface. When an insect walks inside the leaves, the hairs are triggered, causing the leaves to close very rapidly – trapping the animal. The leaves then secrete protease enzymes, which digest the insect’s protein and produce soluble amino acids. These are absorbed by the leaf and used to build new proteins. It is unusual for a photosynthetic plant to show such rapid movement or to gain nourishment other than by photosynthesis. Figure 18.13 Venus flytrap with trapped insect, which will eventually be Figure 18.15 Hare. This animal is a herbivore and is hunted by digested predators such as foxes. Its fur is a good insulator and its colour provides excellent camouflage. The long ears help to pick up and locate sound vibrations. The eyes at the side of the head give the hare good all around vision. The hind legs are very long to enable the animal to run away from predators and its kick is a good defence mechanism. Some species of hare change the colour of their fur in winter from brown to white to provide better camouflage in snow. 276

Adaptive features Key definitions Some plants live in very sandy soil, which does not Adaptive features are the inherited functional features of an retain moisture well. Often this is combined with very low rainfall, making access to water difficult. organism that increase its fitness. Only plants with special adaptations, such as desert Fitness is the probability of that organism surviving and and sand dune species, can survive. C acti reproducing in the environment in which it is found. Cacti are adapted to hot, dry conditions in several ways. Often they have no leaves, or the leaves are Adaptations to arid conditions reduced to spines. This reduces the surface area for transpiration and also acts as a defence against In both hot and cold climates, plants may suffer herbivores. Photosynthesis is carried out by a thick from water shortage. High temperatures accelerate green stem, which offers only a small surface area evaporation from leaves. At very low temperatures for evaporation. Cacti are succulent, i.e. they store the soil water becomes frozen and therefore water in their fleshy tissues and draw on this store for unavailable to the roots of plants. Plants modified to photosynthesis (Figure 18.17). cope with lack of water are called xerophytes. It is thought that the autumn leaf-fall of deciduous trees and shrubs is an essential adaptation to winter ‘drought’. Loss of leaves removes virtually all evaporating surfaces at a time when water may become unavailable. Without leaves, however, the plants cannot make food by photosynthesis and so they enter a dormant condition in which metabolic activity is at a low level. Pine tree The pine tree (Pinus) (Figure 18.16) is an evergreen tree that survives in cold climates. It has small, compact, needle-like leaves. The small surface area of such leaves offers little resistance to high winds. This helps to resist wind damage and can reduce the amount of water lost in transpiration. However, photosynthesis can continue whenever water is available. Sunken stomata create high humidity and reduce transpiration. A thick waxy cuticle is present on the epidermis to prevent evaporation from the surface of the leaf. Figure 18.16  Pine leaves, reduced to needles to lower the rate of Figure 18.17  A cactus (succulent) growing in desert conditions in Arizona transpiration The stomata of many cacti are closed during the day when temperatures are high, and open at night when evaporation is at a minimum. This strategy requires a slightly different form of photosynthesis. At night, carbon dioxide diffuses in through the open stomata and is ‘fixed’ (i.e. incorporated) into an organic acid. Little water vapour is lost at night. In the daytime the stomata are closed but the organic acid breaks down to yield carbon dioxide, which is then built into sugars by photosynthesis. Closure of the stomata in the daytime greatly reduces water loss. 277

18 Variation and selection Marram grass Adaptations to living in water Marram grass (Ammophila) lives on sand dunes (Figure 18.18), where water drains away very Plants adapted to living in water are called quickly. It has very long roots to search for water hydrophytes. An example is the water lily deep down in the sand. Its leaves roll up into straw- (Nymphaea) (Figure 18.20). The leaves contain like tubes in dry weather due to the presence of large air spaces to make them buoyant, so they float hinge cells, which become flaccid as they lose water on or near the surface (Figure 18.21). This enables (Figure 18.19). Leaf rolling, along with the fact that them to gain light for photosynthesis. The lower the stomata are sunken, helps to increase humidity epidermis lacks stomata to prevent water entering around the stomata, reducing transpiration. The the air spaces, while stomata are present on the presence of fine hairs around the stomata reduces air upper epidermis for gas exchange. With land plants, movement so humidity builds up and transpiration most stomata are usually on the lower epidermis. is reduced. The roots of hydrophytes, which can be poorly developed, also contain air spaces. This is because the mud they grow in is poorly oxygenated and the root cells need oxygen for respiration. Stems lack much support as the water they are surrounded by provides buoyancy for the plant. Figure 18.18 Marram grass growing on a sand dune Figure 18.20 Water lily (Nymphaea) Figure 18.19 Transverse section of rolled up Marram grass leaf Figure 18.21 Section through water lily leaf 278

Selection ●● Selection the number of rabbits stemming from the original pair would be 512 (i.e. 2 → 8 → 32 → 128 → 512). Natural selection The population of rabbits, however, remains more or less constant. Many of the offspring in each Theories of evolution have been put forward in generation must, therefore, have failed to survive to various forms for hundreds of years. In 1858, Charles reproductive age. Darwin and Alfred Russel Wallace published a theory of evolution by natural selection, which is still an Figure 18.22  Variation. The garden tiger moths in this picture are all from acceptable theory today. the same family. There is a lot of variation in the pattern on the wings. The theory of evolution by natural selection is as Competition and selection follows: There will be competition between members of ● Individuals within a species are all slightly different the rabbit population for food, burrows and mates. If food is scarce, space is short and the number of from each other (Figure 18.22). These differences potential mates limited, then only the healthiest, are called variations. most vigorous, most fertile and otherwise well- ● If the climate or food supply changes, individuals adapted rabbits will survive and breed. possessing some of these variations may be better able to survive than others. For example, a variety The competition does not necessarily involve of animal that could eat the leaves of shrubs as well direct conflict. The best adapted rabbits may be as grass would be more likely to survive a drought able to run faster from predators, digest their food than one that fed only on grass. more efficiently, have larger litters or grow coats that ● If one variety lives longer than others, it is also camouflage them better or more effectively reduce likely to leave behind more offspring. A mouse heat losses. These rabbits will survive longer and that lives for 12 months may have ten litters of five leave more offspring. If the offspring inherit the babies (50 in all). A mouse that lives for 6 months advantageous characteristics of their parents, they may may have only five litters of five babies (25 in all). give rise to a new race of faster, different coloured, ● If some of the offspring inherit alleles responsible for the variation that helped the parent survive better, they too will live longer and have more offspring. ● In time, this particular variety will outnumber and finally replace the original variety. This is sometimes called ‘the survival of the fittest’. However, ‘fitness’, in this case, does not mean good health but implies that the organism is well fitted to the conditions in which it lives. Thomas Malthus, in 1798, suggested that the increase in the size of the human population would outstrip the rate of food production. He predicted that the number of people would eventually be regulated by famine, disease and war. When Darwin read the Malthus essay, he applied its principles to other populations of living organisms. He observed that animals and plants produce vastly more offspring than can possibly survive to maturity and he reasoned that, therefore, there must be a ‘struggle for survival’. For example, if a pair of rabbits had eight offspring that grew up and formed four pairs, eventually having eight offspring per pair, in four generations 279

18 Variation and selection thicker furred and more fertile rabbits, which gradually Although this is an attractive and plausible replace the original, less well-adapted varieties. The hypothesis of how natural selection could occur, new variations are said to have survival value. some of the evidence does not support the hypothesis or has been called into question. This is natural selection; the better adapted varieties are ‘selected’ by the pressures of the For example, the moths settle most frequently on environment (selection pressures). the underside of branches rather than conspicuously on tree trunks, as in Figure 18.23. Also, in several For natural selection to be effective, the variations unpolluted areas the dark form is quite abundant, have to be heritable. Variations that are not heritable for example 80% in East Anglia in England. Research are of no value in natural selection. Training may give is continuing in order to test the hypothesis. athletes more efficient muscles, but this characteristic will not be passed on to their children. Selective breeding The peppered moth A possible example of natural selection is provided by The process of selective breeding involves humans a species of moth called the peppered moth, found selecting individuals with desirable features. These in Great Britain. The common form is speckled but individuals are then cross-bred to produce the there is also a variety that is black. The black variety next generation. Offspring with the most desirable was rare in 1850, but by 1895 in the Manchester features are chosen to continue the breeding area of England its numbers had risen to 98% of the programme and the process is repeated over a population of peppered moths. Observation showed number of generations. that the light variety was concealed better than the dark variety when they rested on tree-trunks covered Human communities practise this form of selection with lichens (Figure 18.23). In the Manchester area when they breed plants and animals for specific of England, pollution had caused the death of the characteristics. The many varieties of cat that you see lichens and the darkening of the tree-trunks with today have been produced by selecting individuals soot. In this industrial area the dark variety was the with pointed ears, particular fur colour or length, or better camouflaged (hidden) of the two and was even no tail, etc. One of the kittens in a litter might not picked off so often by birds. So the dark variety vary from the others by having distinctly pointed ears. survived better, left more offspring and nearly This individual, when mature, is allowed to breed. replaced the light form. From the offspring, another very pointed-eared variant is selected for the next breeding stock, and so on, until The selection pressure, in this case, was presumed the desired or ‘fashionable’ ear shape is established in a to be mainly predation by birds. The adaptive true-breeding population (Figure 18.24). variation that produced the selective advantage was the dark colour. More important are the breeding programmes to improve agricultural livestock or crop plants. Animal- breeders will select cows for their high milk yield and (a) (b) (c) (d) Figure 18.23  Selection for varieties of the peppered moth 280

Selection sheep for their wool quality. Plant-breeders will select varieties for their high yield and resistance to fungus diseases (Figure 18.25). Figure 18.24  Selective breeding. The Siamese cat, produced by artificial Figure 18.25  Selective breeding in tomatoes. Different breeding selection over many years programmes have selected genes for fruit size, colour and shape. Similar processes have given rise to most of our cultivated plants and domesticated animals. Evolution Selective breeding Key definitions An important part of any breeding programme is Adaptation is the process, resulting from natural selection, the selection of the desired varieties. The largest fruit on a tomato plant might be picked and its seeds by which populations become more suited to their planted next year. In the next generation, once again environment over many generations. only seeds from the largest tomatoes are planted. Evolution can be described as the change in adaptive features Eventually it is possible to produce a true-breeding of a population over time as a result of natural selection. variety of tomato plant that forms large fruits. Figure 18.25 shows the result of such selective breeding. Most biologists believe that natural selection, among The same technique can be used for selecting other other processes, contributes to the evolution of new desirable qualities, such as flavour and disease species and that the great variety of living organisms resistance. on the Earth is the product of millions of years of evolution involving natural selection. Similar principles can be applied to farm animals. A ntibiotic-resistant bacteria Desirable characteristics, such as high milk yield Antibiotics are drugs used to treat infections caused and resistance to disease, may be combined. Stock- by bacteria (see ‘Medicinal drugs’ in Chapter 15). breeders will select calves from cows that give large Bacterial cells reproduce very rapidly, perhaps as quantities of milk. These calves will be used as often as once every 20 minutes. Thus a mutation, breeding stock to build a herd of high yielders. A even if it occurs only rarely, is likely to appear in characteristic such as milk yield is probably under a large population of bacteria. If a population the control of many genes. At each stage of selective of bacteria containing one or two drug-resistant breeding the farmer, in effect, is keeping the mutants is subjected to that particular drug, the non- beneficial genes and discarding the less useful genes resistant bacteria will be killed but the drug-resistant from his or her animals. mutants survive (Figure 15.1). Mutant genes are inherited in the same way as normal genes, so when Selective breeding in farm stock can be slow and the surviving mutant bacteria reproduce, all their expensive because the animals often have small offspring will be resistant to the drug. numbers of offspring and breed only once a year. By producing new combinations of genes, selective breeding achieves the same objectives as 281

18 Variation and selection genetic engineering but it takes much longer and is that provide an advantage, to cope with changes less predictable. in environmental conditions for example, are more likely to survive, while others die before they can In selective breeding, the transfer of genes takes breed and pass on their genes. However, variation place between individuals of the same or closely within the population remains. related species. Genetic engineering involves transfer between unrelated species. Artificial selection is used by humans to produce varieties of animals and plants that have an increased Selective breeding and genetic engineering economic importance. It is considered a safe way of both endeavour to produce new and beneficial developing new strains of organisms, compared with combinations of genes. Selective breeding, however, genetic engineering, and is a much faster process than is much slower and less precise than genetic natural selection. However, artificial selection removes engineering. On the other hand, cross-breeding variation from a population, leaving it susceptible techniques have been around for a very long time to disease and unable to cope with changes in and are widely accepted. environmental conditions. Potentially, therefore, artificial selection puts a species at risk of extinction. One of the drawbacks of selective breeding is that the whole set of genes is transferred. As well as the desirable genes, there may be genes that, in a homozygous condition, would be harmful. It is known that artificial selection repeated over a large number of generations tends to reduce the fitness of the new variety. A long-term disadvantage of selective breeding is the loss of variability. By eliminating all the offspring that do not bear the desired characteristics, many genes are lost from the population. At some future date, when new combinations of genes are sought, some of the potentially useful ones may no longer be available. In attempting to introduce, in plants, characteristics such as salt tolerance or resistance to disease or drought, the geneticist goes back to wild varieties, as shown in Figure 18.26. However, with the current rate of extinction, this source of genetic material is diminishing. In the natural world, reduction of variability could lead to local extinction if the population was unable to adapt, by natural selection, to changing conditions. Comparing natural and artificial (a) (b) (c) (d) (e) selection Figure 18.26  The genetics of bread wheat. A primitive wheat (a) was Natural selection occurs in groups of living crossed with a wild grass (b) to produce a ­better-yielding hybrid wheat organisms through the passing on of genes to the (c). The hybrid wheat (c) was crossed with another wild grass (d) to next generation by the best adapted organisms, produce one of the varieties of wheat (e) which is used for making flour without human interference. Those with genes and bread. 282

selection Questions Extended 4 Suggest some good characteristics that an animal-breeder Core 1 Study the following photographs and captions, then make a might try to combine in sheep by mating different varieties together. list of the adaptations of each animal. 5 A variety of barley has a good ear of seed but has a long a long-eared bat (Figure 18.14) stalk and is easily blown over. Another variety has a short, b hare (Figure 18.15) sturdy stalk but a poor ear of seed. b polar bear (Figure 18.11) (See also details in the text.) Suggest a breeding programme to obtain and select a new 2 What features of a bird’s appearance and behaviour do you variety that combines both of the useful characteristics. think might help it compete for a mate? Choose letters to represent the genes and show the 3 What selection pressures do you think might be operating genotypes of the parent plants and their offspring. on the plants in a lawn? Checklist Adaptive features After studying Chapter 18 you should know and understand the • An adaptive feature is an inherited feature that helps an following: organism to survive and reproduce in its environment. Variation • Adaptive features of a species can be recognised from its image in a drawing or photograph. • Variation is the differences between individuals of the same species. • An adaptive feature is the inherited functional features of an organism that increase its fitness. • Variations within a species may be inherited or acquired. • Continuous variation results in a range of phenotypes • Fitness is the probability of that organism surviving and reproducing in the environment in which it is found. between two extremes, e.g. height in humans. • Discontinuous variation results in a limited number of • Hydrophytes are plants that have adaptive features to live in a watery environment. phenotypes with no intermediates, e.g. tongue rolling. • Mutation is the way in which new alleles are formed. • Xerophytes are plants that have adaptive features to live • Increases in the rate of mutation can be caused by ionising in very dry environments. radiation and some chemicals. Selection • Discontinuous variation results, usually, from the effects • Some members of a species may have variations that enable of a single pair of alleles, and produces distinct and them to compete more effectively. consistent differences between individuals. • These variants will live longer and leave more offspring. • Blood groups are an example of discontinuous variation. • If the beneficial variations are inherited, the offspring will • Discontinuous variations cannot be changed by the also survive longer. environment. • The new varieties may gradually replace the older varieties. • Phenotypic (continuous) variations are usually • Natural selection involves the elimination of less well-adapted controlled by a number of genes affecting the varieties by environmental pressures. same characteristic and can be influenced by the • Selective breeding is used to improve commercially useful environment. • A gene mutation is a change in the base sequence plants and animals. of DNA. • Sickle-cell anaemia is caused by a change in the base • Adaptation is the process, resulting from natural selection, sequence of the gene for haemoglobin. This results in by which populations become more suited to their abnormal haemoglobin, which changes shape when environment over many generations. oxygen levels are low. • The inheritance of sickle-cell anaemia can be predicted • The development of strains of antibiotic-resistant bacteria using genetic diagrams. is an example of natural selection. • People who are heterozygous for the sickle-cell allele have a resistance to malaria. • Selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals. • Evolution is the change in adaptive features of a population over time as the result of natural selection. 283

19 Organisms and their environment Energy flow ●● Energy ﬂow Sun as source of energy Nearly all living things depend on the Sun to provide Flow of energy through organisms energy. This is harnessed by photosynthesising plants and the energy is then passed through food chains. Food chains and food webs Define food chain, food web, producer, consumer, herbivore, Dependence on sunlight carnivore, decomposer Interpret food chains, food webs and pyramids of number With the exception of atomic energy and tidal Impact of over-harvesting and introduction of foreign species power, all the energy released on Earth is derived on food chains and webs from sunlight. The energy released by animals comes, ultimately, from plants that they or their Transfer of energy between trophic levels prey eat and the plants depend on sunlight for Define trophic level making their food. Photosynthesis is a process Loss of energy between levels in which light energy is trapped by plants and Efficiency of supplying green plants as human food converted into chemical energy (stored in molecules Identify levels in food chains, webs, pyramids of number such as carbohydrates, fats and proteins). Since all and biomass animals depend, in the end, on plants for their food, Describe and interpret pyramids of biomass they therefore depend indirectly on sunlight. A few Advantages of using pyramids of biomass examples of our own dependence on photosynthesis Recycling are given below. Nutrient cycles sunlight Carbon cycle Water cycle photosynthesis photosynthesis photosynthesis in wheat in grass flowering plants Nitrogen cycle Roles of micro-organisms in nitrogen cycle wheat grains cow nectar Population size flour milk bees Define population Factors affecting rate of population growth bread cheese honey Human population growth Define community, ecosystem Factors affecting the increase in size of the human population Identify and explain phases on a sigmoid population growth curve Nearly all the energy released on the Earth can be traced back to sunlight. Coal comes from tree- like plants, buried millions of years ago. These plants absorbed sunlight for their photosynthesis when they were alive. Petroleum was formed, also millions of years ago, probably from the partly decayed bodies of microscopic algae that lived in the sea. These, too, had absorbed sunlight for photosynthesis. 284

Food chains and food webs Today it is possible to use mirrors and solar sugar from sugar-cane can be fermented to alcohol, panels to collect energy from the Sun directly, but and used as a motor fuel instead of petrol. the best way, so far, of trapping and storing energy from sunlight is to grow plants and make use of Eventually, through one process or another, all the their products, such as starch, sugar, oil, alcohol and chemical energy in organisms is transferred to the wood, for food or as energy sources. For example, environment. However, it is not a cyclical process like those described later in this chapter. ●● Food chains and food webs Key definitions A food chain shows the transfer of energy from one organism to the next, beginning with a producer. A food web is a network of interconnected food chains. A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. A consumer is an organism that gets its energy from feeding on other organisms. A herbivore is an animal that gets its energy by eating plants. A carnivore is an animal that gets its energy by eating other animals. A decomposer is an organism that gets its energy from dead or waste organic material. ‘Interdependence’ means the way in which living organisms depend on each other in order to remain alive, grow and reproduce. For example, bees depend for their food on pollen and nectar from flowers. Flowers depend on bees for pollination (Chapter 16). Bees and flowers are, therefore, interdependent. Food chains Figure 19.1  A food chain. The caterpillar eats the leaf; the blue tit eats the caterpillar but may fall prey to the kestrel. One important way in which organisms depend on each other is for their food. Many animals, such the chain are often large and few in number. The food as rabbits, feed on plants. Such animals are called pyramids in Figure 19.2 show this relationship. There herbivores. Animals that eat other animals are called will be millions of microscopic, single-celled algae in carnivores. A predator is a carnivore that kills and a pond (Figure 19.3(a)). These will be eaten by the eats other animals. A fox is a predator that preys larger but less numerous water fleas and other crustacea on rabbits. Scavengers are carnivores that eat the (Figure 19.3(b)), which in turn will become the food of dead remains of animals killed by predators. These small fish such as minnow and stickleback. The hundreds are not hard and fast definitions. Predators will of small fish may be able to provide enough food for sometimes scavenge for their food and scavengers only four or five large carnivores, like pike or perch. may occasionally kill living animals. Animals obtain their energy by ingestion. The organisms at the base of the food pyramids in Figure 19.2 are plants. Plants produce food from Basically, all animals depend on plants for their food. carbon dioxide, water and salts (see ‘Photosynthesis’, Foxes may eat rabbits, but rabbits feed on grass. A hawk Chapter 6), and are, therefore, called producers. eats a lizard, the lizard has just eaten a grasshopper The animals that eat the plants are called primary but the grasshopper was feeding on a grass blade. This consumers, e.g. grasshoppers. Animals that prey on relationship is called a food chain (Figure 19.1). the plant-eaters are called secondary consumers, e.g. shrews, and these may be eaten by tertiary The organisms at the beginning of a food chain are consumers, e.g. weasels or kestrels (Figure 19.4). usually very numerous while the animals at the end of 285

19 Organisms and their envirOnment perch consumers minnows producers owl water fleas blue tits microscopic algae caterpillars (b) water plant leaves (a) land Figure 19.2 Examples of food pyramids (pyramids of numbers) (pfaoh)oydtofppohpoyylrdataonpmpkyltairdoanmnkinti(od×tnhi1n(e0×t01hw)0eaT0wth)eaTerthseeer.sme imcricorsocsocpoipcicaalglgaaeeffoorrmm tthhee bbaassisisooffa (zbo)opzloaonpkltaonnkt(o×n2(0×)2T0h) eTsheescerucsrtuasctaecaeawwillilel eaattmmicicrroossccooppiicc aallggaaee. Figure 19.3 Plankton. The microscopic organisms that live in the surface waters of the sea or fresh water are called, collectively, plankton. The single-celled algae (see Chapter 1) are the phytoplankton. They are surrounded by water, salts and dissolved carbon dioxide. Their chloroplasts absorb sunlight and use its energy for making food by photosynthesis. Phytoplankton is eaten by small animals in the zooplankton, mainly crustacea (see Chapter 1). Small fish will eat the crustacea. Pyramids of numbers The width of the bands in Figure 19.2 is meant to represent the relative number of organisms at each trophic level. So the diagrams are sometimes called pyramids of numbers. However, you can probably think of situations where a pyramid of numbers would not show the same effect. For example, a single sycamore tree may provide food for thousands of greenfly. One oak tree may feed hundreds of caterpillars. In these cases the pyramid of numbers is upside-down, as shown in Figure 19.5. Figure 19.4 The kestrel, a secondary or tertiary consumer Food webs Food chains are not really as straightforward as described above, because most animals eat more than one type of food. A fox, for example, does not feed entirely on rabbits but takes beetles, rats and voles in 286

Food chains and food webs its diet. To show these relationships more accurately, nearly all the rabbits in England. Foxes ate more a food web can be drawn up (Figure 19.6). voles, beetles and blackberries, and attacks on lambs and chickens increased. Even the vegetation was fleas quaternary consumer affected because the tree seedlings that the rabbits used to nibble on were able to grow. As a result, owl tertiary consumer woody scrubland started to develop on what had been grassy downs. A similar effect is shown in blue tits secondary consumer Figure 19.7. caterpillars primary consumer The effects of over-harvesting oak Over-harvesting causes the reduction in numbers of tree producer a species to the point where it is endangered or made extinct. As a result biodiversity is affected. The species Figure 19.5  An inverted pyramid of numbers may be harvested for food, or for body parts such as tusks (elephants), horns (rhinos – Figure 19.8), The food webs for land, sea and fresh water, or for bones and fur (tigers) or for selling as pets (reptiles, ponds, rivers and streams, will all be different. Food birds and fish, etc.). In parts of Africa, bush meat webs will also change with the seasons when the food is used widely as a source of food. Bush meat is the supply changes. flesh of primates, such as monkeys. However, hunting If some event interferes with a food web, all the these animals is not always regulated or controlled organisms in it are affected in some way. For and rare species can be threatened as a result of example, if the rabbits in Figure 19.6 were to die indiscriminate killing. (See also ‘Habitat destruction’ out, the foxes, owls and stoats would eat more in Chapter 21.) beetles and rats. Something like this happened in 1954 when the disease myxomatosis wiped out fox stoat owl beetle rat rabbit Figure 19.6  A food web vegetation 287

19 Organisms and their envirOnment (Sah)eeSphheaepveheavaeteenataennyasneyesdeleindlginsgws hthicaht ggrreewwuunnddererthtehetretereses (stTbehee)endltyiTrneeeganesryssheelaaaevrtdesellgrian,rtotgehwrs,enhthfaeevnefcegenrchoeawhsnaksekpetptthteheshsheeeeppooffff aanndd the tree Figure 19.7 Effect of grazing 300 thousand tonnes 200 100 Figure 19.8 The rhinoceros is endangered because some people believe, 0 mistakenly, that powdered rhino horn (Cornu Rhinoceri Asiatici) has medicinal 1970 72 74 76 78 80 82 84 86 88 1990 properties, and others greatly prize rhino horn handles for their daggers. year Overﬁshing Figure 19.9 Landings of North Sea cod from 1970 to 1990 Small populations of humans, taking fish from lakes per day at sea goes down even more. Eventually the or oceans and using fairly basic methods of capture, stocks are so depleted that it is no longer economical had little effect on fish numbers. At present, however, to exploit them. The costs of the boats, the fuel and commercial fishing has intensified to the point the wages of the crew exceed the value of the catch. where some fish stocks are threatened or can no Men are laid off, boats lie rusting in the harbour and longer sustain fishing. In the past 100 years, fishing the economy of the fishing community and those fleets have increased and the catching methods have who depend on it is destroyed. Overfishing has become more sophisticated. severely reduced stocks of many fish species: herring in the North Sea, halibut in the Pacific and anchovies If the number of fish removed from a population off the Peruvian coast, for example. In 1965, exceeds the number of young fish reaching maturity, 1.3 million tonnes of herring were caught in the then the population will decline (Figure 19.9). North Sea. By 1977 the catch had diminished to 44 000 tonnes, i.e. about 3% of the 1965 catch. At first, the catch size remains the same but it takes longer to catch it. Then the catch starts to contain Similarly, whaling has reduced the population a greater number of small fish so that the return of many whale species to levels that give cause for concern. Whales were the first marine organisms to face extinction through overfishing. This happened 288

Food chains and food webs in the early 1800s when they were killed for their reptiles and tortoises, along with young animals. blubber (a thick fat layer around the body of the The Galapagos Islands provide a habitat for many mammal) for use as lamp oil. The blue whale’s rare species, which became endangered as a result numbers have been reduced from about 2 000 000 of the presence of the rats. A programme of rat to 6000 as a result of intensive hunting. extermination is now being carried out on the islands to protect their unique biodiversity. Overfishing can reduce the populations of fish species and can also do great damage to the The prickly pear cactus, Opuntia, was introduced environment where they live. For example, the use of to Australia in 1839 for use as a living fence to heavy nets dragged along the sea floor to catch the fish control the movement of cattle, but its growth got can wreck coral reefs, destroying the habitats of many out of control because of the lack of herbivores that other animal species. Even if the reef is not damaged, eat it. Millions of acres of land became unusable. fishing for the top predators such as grouper fish has A moth, Cactoblastis cactorum, whose young feed a direct effect on the food chain: fish lower down the on the cactus, was successfully introduced from chain increase in numbers, and overgraze on the reef. Argentina and helped to control the spread of This process is happening on the Great Barrier Reef in the cactus. Other places with similar problems, Australia. Grouper fish are very slow growing and take for example the island of Nevis in the West a long time to become sexually mature, so the chances Indies, followed Australia’s example, but with of them recovering from overfishing are low and they less successful results. The moth had no natural are becoming endangered. predators and ate other native cactus species as well as the prickly pear, bringing them to the brink of Introducing foreign species to a extinction. The moth is now spreading to parts of habitat the United States of America and poses a threat to other cactus species. One of the earliest examples of this process was the accidental introduction of rats to the Galapagos Food chains and webs can also be disrupted by Islands by pirates or whalers in the 17th or 18th the use of pesticides and other poisons, sometimes centuries. The rats had no natural predators and released accidentally during human activities. More food was plentiful: they fed on the eggs of birds, details can be found in Chapter 21. Energy transfer 40% warms up the plants, the soil and the air, leaving only about 1% to be used in photosynthesis Study Figure 19.1. When an herbivorous animal for making new organic matter in the leaves of the eats a plant (the caterpillar feeding on a leaf), the plants (Figure 19.10). chemical energy stored in that plant leaf is transferred to the herbivore. Similarly, when a carnivore (the This figure of 1% will vary with the type of blue tit) eats the herbivore, the carnivore gains the vegetation being considered and with climatic energy stored in the herbivore. If the carnivore is factors, such as availability of water and the eaten by another carnivore (the kestrel), the energy is soil temperature. Sugar-cane grown in ideal transferred again. conditions can convert 3% of the Sun’s energy into photosynthetic products; sugar-beet at the height of U se of sunlight its growth has nearly a 9% efficiency. Tropical forests To try and estimate just how much life the Earth can and swamps are far more productive than grassland support it is necessary to examine how efficiently but it is difficult, and, in some cases undesirable, to the Sun’s energy is used. The amount of energy harvest and utilise their products. from the Sun reaching the Earth’s surface in 1 year ranges from 2 million to 8 million kilojoules per In order to allow crop plants to approach m2 (2–8 ×109 J m–2 yr–1) depending on the latitude. their maximum efficiency they must be provided When this energy falls onto grassland, about with sufficient water and mineral salts. This can 20% is reflected by the vegetation, 39% is used in be achieved by irrigation and the application of evaporating water from the leaves (transpiration), fertiliser. 289

19 Organisms and their environment SUNLIGHT 20% 39% evaporates water 30% lost as energy 39% from plant 10% for 40% growth 20% reflected from plant FOOD 60% not digested Figure 19.11  Energy transfer from plants to animals only 1% used in 40% warms up soil, be represented by classifying the organisms in a photosynthesis air and vegetation community as producers, or primary, secondary or tertiary consumers, and showing their relative masses Figure 19.10  Absorption of Sun’s energy by plants in a pyramid such as the one shown in Figure 19.2 but on a more accurate scale. In Figure 19.12 the width E nergy transfer between organisms of the horizontal bands is proportional to the masses Having considered the energy conversion from (dry weight) of the organisms in a shallow pond. sunlight to plant products, the next step is to study the efficiency of transmission of energy from plant 0.1 tertiary consumers products to primary consumers. On land, primary 0.66 secondary consumers consumers eat only a small proportion of the available vegetation. In a deciduous forest only about 1.25 primary consumers 2% is eaten; in grazing land, 40% of the grass may be eaten by cows. In open water, however, where the 17.7 producers producers are microscopic plants (phytoplankton, see Figure 19.3(a)) and are swallowed whole by the Figure 19.12  Biomass (dry weight) of living organisms in a primary consumers in the zooplankton (see Figure shallow pond (grams per square metre) 19.3(b)), 90% or more may be eaten. In the land communities, the parts of the vegetation not eaten Key definitions by the primary consumers will eventually die and be The trophic level of an organism is its position in a food used as a source of energy by the decomposers. chain, food web or pyramid of numbers or biomass. A cow is a primary consumer; over 60% of the grass it eats passes through its alimentary canal It is very unusual for food chains to have more than (Chapter 7) without being digested. Another 30% is five trophic levels because, on average, about 90% of used in the cow’s respiration to provide energy for its the energy is lost at each level. Consequently, very little movement and other life processes. Less than 10% of of the energy entering the chain through the producer the plant material is converted into new animal tissue is available to the top consumer. The food chain below to contribute to growth (Figure 19.11). This figure shows how the energy reduces through the chain. It is will vary with the diet and the age of the animal. In a based on grass obtaining 100 units of energy. fully grown animal all the digested food will be used for energy and replacement and none will contribute grass → locust → lizard → snake → mongoose to growth. Economically it is desirable to harvest the primary consumers before their rate of growth starts 100 10 1 0.1 0.01 to fall off. units units unit unit unit The transfer of energy from primary to secondary Energy transfer in agriculture consumers is probably more efficient, since a greater In human communities, the use of plant products proportion of the animal food is digested and to feed animals that provide meat, eggs and dairy absorbed than is the case with plant material. The products is wasteful, because only 10% of the plant transfer of energy at each stage in a food chain may 290

Food chains and food webs material is converted to animal products. It is more Pyramids of biomass economical to eat bread made from the wheat As stated earlier, displaying food chains using than to feed the wheat to hens and then eat the pyramids of number, such as those shown in eggs and chicken meat. This is because eating the Figure 19.5, can produce inverted pyramids. This is wheat as bread avoids using any part of its energy because the top consumers may be represented by to keep the chickens alive and active. Energy losses large numbers of very small organisms, for example, can be reduced by keeping hens indoors in small fleas feeding on an owl. The way around this problem cages, where they lose little heat to the atmosphere is to consider not the single tree, but the mass of the and cannot use much energy in movement leaves that it produces in the growing season, and the (Figure 19.13). The same principles can be applied mass of the insects that can live on them. Biomass in ‘intensive’ methods of rearing calves. However, is the term used when the mass of living organisms many people feel that these methods are less than is being considered, and pyramids of biomass can be humane, and the saving of energy is far less than if constructed as in Figure 19.12. A pyramid of biomass the plant products were eaten directly by humans, as is nearly always the correct pyramid shape. is the case in vegetarians. An alternative is to calculate the energy available in a year’s supply of leaves and compare this with the energy needed to maintain the population of insects that feed on the leaves. This would produce a pyramid of energy, with the producers at the bottom having the greatest amount of energy. Each successive trophic level would show a reduced amount of energy. The elements that make up living organisms are recycled, i.e. they are used over and over again (see next section). This is not the case with energy, which flows from producers to consumers and is eventually lost to the atmosphere as heat. Figure 19.13  Battery chickens. The hens are well fed but kept in Recycling crowded and cramped conditions with no opportunity to move about or scratch in the soil as they would normally do. There are a number of organisms that have not been fitted into the food webs or food Consideration of the energy flow of a modern chains described so far. Among these are the agricultural system reveals other sources of decomposers. Decomposers do not obtain their inefficiency. To produce 1 tonne of nitrogenous food by photosynthesis, nor do they kill and eat fertiliser takes energy equivalent to burning 5 tonnes living animals or plants. Instead they feed on dead of coal. Calculations show that if the energy needed and decaying matter such as dead leaves in the to produce the fertiliser is added to the energy used soil or rotting tree-trunks (Figure 19.14). The to produce a tractor and to power it, the energy most numerous examples are the fungi, such as derived from the food so produced is less than that mushrooms, toadstools or moulds, and the bacteria, expended in producing it. particularly those that live in the soil. They produce extracellular enzymes that digest the decaying matter and then they absorb the soluble products back into their cells. In so doing, they remove the dead remains of plants and animals, which would otherwise collect on the Earth’s surface. They also break these remains down into substances that can be used by other organisms. Some bacteria, for example, break down the protein of dead plants and animals and release nitrates, which are taken up by 291