geneticsstudy of heredity in general and of genes in particular.

Since prehistoric times, man the dawn of civilization, mankind has recognized the influence of heredity and has applied its principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than 6,000 years old, for example, shows pedigrees of horses and indicates possible inherited characteristics; other . Other old carvings show cross-pollination of date palm trees. Most of the mechanisms of heredity, however, remained a mystery until the 20th 19th century, when scientifically supported information became available.Genetics genetics as a systematic science began.

Genetics arose out of the identification of genes, the fundamental units responsible for heredity. Genetics may be defined as the study of the way in which genes operate and the way genes at all levels, including the ways in which they act in the cell and the ways in which they are transmitted from parents to offspring. Modern genetics involves study of the mechanism of gene action—the way in which the genetic material (focuses on the chemical substance that genes are made of, called deoxyribonucleic acid, or DNA) affects physiological reactions , and the ways in which it affects the chemical reactions that constitute the living processes within the cell. Although genes determine the features an individual may develop, the features that actually develop depend upon the complex interaction between genes and their environment. Normal green Gene action depends on interaction with the environment. Green plants, for example, have genes containing the information necessary to synthesize the photosynthetic pigment chlorophyll that gives them their green colour, and chlorophyll . Chlorophyll is synthesized in an environment containing light ; i.e., because the gene for chlorophyll is expressed only when it interacts with light. If the a plant is placed in a dark environment, chlorophyll synthesis stops ; i.e., because the gene is no longer expressed.

Genetics overlaps many different branches of biology and many other sciences; e.g., chemistry, physics, mathematics, sociology, psychology, and medicine. Microbiologists who study inheritance in microorganisms are called microbial geneticists; cytologists who study the genetics of cells are called cytogeneticists. Biochemical, or molecular, geneticists investigate the chemical nature of the gene and its methods of action. Some physicists have applied their techniques to molecular genetics, and mathematicians may specialize in population genetics. Behavioral scientists also look to genetics to solve certain problems of human and animal behaviour. Specialists in medical genetics or genetic counselling act on the knowledge that many of man’s afflictions are hereditary.

Historical background

The Greek philosopher Pythagoras speculated around 500 BC that human life begins with a blend of male and female fluids, or semens, originating in body parts. Aristotle later postulated that the semens are purified blood and that blood, therefore, is the element of heredity. That this later concept persisted in the Western world is indicated by such common phrases as blue blood, blood-will-tell, blood relative, bad blood, and royal blood.

About 1651, William Harvey disproved the Greek concept; his discovery that deer embryos have the appearance of a tiny ball during early developmental stages and resemble a deer only later in development led him to conclude that the origin of the tiny ball was a small egg. Before the end of the 17th century, it had been suggested that the female structures called ovaries are the source of eggs and that sperm might carry the hereditary material of the male.

Early in the 19th century, Jean-Baptiste Lamarck suggested that acquired characteristics are inherited. Around 1865 Gregor Mendel reported his discoveries on inheritance in garden peas. A few years later, the DNA component of genes was isolated from pus cells, and it was discovered that salmon sperm also contain considerable amounts of DNA. Late in the 19th century, a German physician, August Weismann, showed that reproductive cells (germ plasm) are independent of other body cells (somatoplasm), thus refuting earlier hypotheses of inheritance of acquired characteristics.

The concept of sudden changes in heredity (mutations) was introduced in the beginning of the 20th century. Discoveries concerning sex determination in insect chromosomes and gene linkage on a chromosome of sweet peas were made soon afterward in the United States and England. In 1908 an English mathematician and a German physician formulated the so-called Hardy–Weinberg principle, which provided the foundation for population genetics. The study of biochemical genetics was begun in 1909 in England with an effort to discover the way by which gene-induced enzyme deficiencies cause abnormalities.

Hermann J. Muller, a U.S. geneticist, induced mutations in the fruit fly with X rays in 1927. Experiments on the mold Neurospora by George W. Beadle and Edward L. Tatum proved that the function of most genes is to direct the synthesis of enzymes, which thus are the expression of many hereditary traits. By 1944 DNA had been proved to be the substance of heredity, and in 1953 James D. Watson and F.H.C. Crick reported a structure of DNA compatible with the capability for self-duplication. Two French Nobel Prize winners, François Jacob and Jacques Monod, discovered the mechanism by which hereditary information is transferred from genes to the site of protein (enzyme) synthesis. Their work resulted in the discovery of the genetic code, by which DNA is translated into protein. Barbara McClintock, who received a Nobel Prize in 1983, was cited for her discovery of mobile genetic elements—some of the mechanisms that account for mutation.



Historical background
Ancient theories of pangenesis and blood in heredity





Preformation and natural selection



The work of Mendel




How the gene idea became reality


The discovery of linked genes


Early molecular genetics


DNA and the genetic code



Recombinant DNA technology and the polymerase chain reaction



Areas of study
Classical genetics

Classical genetics, which remains a basis the foundation for all other topics areas in genetics, is concerned primarily with the method by which genetic traits classified traits—classified as dominant (always expressed), recessive (subordinate to a dominant trait), intermediate (partially expressed), or polygenic (due to multiple genes) are —are transmitted in plants and animals. These traits may be sex-linked (result resulting from the action of a gene on the sex, or X, chromosome) or autosomal (result resulting from the action of a gene on a chromosome other than a sex chromosome). Classical genetics began with Mendel’s study of inheritance in garden peas and continues with studies of inheritance in many different plants and animals. Today a prime reason for performing classical genetics is for gene discovery—the finding and assembling of a set of genes that affects a biological property of interest.


Cytogenetics, the microscopic study of chromosomes, blends the skills of cytologists, who study the structure and activities of cells, with those of geneticists, who study the relationship between the mechanism of heredity and cellular activitiesgenes. Cytologists discovered chromosomes and the way in which they duplicate and separate during cell division at about the same time that geneticists began to understand the behaviour of genes at the cellular level. The close correlation between the two disciplines led to their combination.



Microbial genetics

Microorganisms were generally ignored by the early geneticists because they are small in size and were thought to lack variable traits and the sexual reproduction necessary for a mixing of genes from different organisms. After it was discovered that microorganisms can have many different physical and physiological characteristics and also that are able amenable to reproduce sexuallystudy, they became objects of great interest to geneticists because of their small size and the fact that they reproduce much more rapidly than larger organisms; i.e., a mutation, or change, occurs in a gene about one time in 10,000,000 gene duplications, and one bacterium may produce 10,000,000,000 offspring, among which are numerous mutants, in 48 hours.

Many discoveries in microbial genetics have been applied to other areas of genetics; for example, the way in which genes produce enzymes that function in turn to produce genetic traits has important applications to human genetics. Much of microbial genetics also applies to the study of the genetics of viruses.

Molecular genetics

Molecular genetics includes the study of the molecular nature of the gene and the method by which genes control the activities of the cell. Molecular geneticists have studied the molecular structure of a gene (e.g., that involved in the synthesis of the human blood pigment, hemoglobin) and determined the exact sequence of its components; in addition, they have created a synthetic gene by joining the components comprising a known gene in the correct sequence. Genetic engineering had become a commercial enterprise by the early 1980s.

Population genetics

A study of genes in populations of animals . Bacteria became important model organisms in genetic analysis, and many discoveries of general interest in genetics arose from their study. Bacterial genetics is the centre of cloning technology.


Molecular genetics




Population genetics



Human population geneticists have traced the origins and migration and invasion routes of man; genetic studies of present-day Europeans, for example, reveal routes of human migrations that occurred hundreds or thousands of years ago. The origin of the people inhabiting South Pacific islands and the degree of intermingling among mixed races also are studied by human population geneticists.



Behaviour genetics

Another aspect of genetics is the study of the influence of heredity on behaviour. Many characteristics once considered to be acquired behavioral patterns actually are of a hereditary nature. The role of heredity in instinctive patterns of behaviour among animals has long been recognized, but many of man’s actions also have a hereditary explanation. The effect of various drugs (e.g., the hallucinogenic drug lysergic acid diethylamide, or LSD) on behavioral patterns in animals, including humans, is of particular interestaspects of animal behaviour are genetically determined and can therefore be treated as similar to other biological properties. This is the subject material of behaviour genetics, whose goal is to determine which genes control various aspects of behaviour in animals. Human behaviour is difficult to analyze because of the powerful effects of environmental factors, such as culture. Few cases of genetic determination of complex human behaviour are known. Genomics studies provide a useful way to explore the genetic factors involved in complex human traits such as behaviour.

Human genetics

Some geneticists specialize in the hereditary processes of human genetics. When classical geneticists first determined the principles of heredity in plants, fruit flies, mice, and other forms of life, they tried to interpret man’s heredity in a similar way but found many traits that did not fit the patterns. As techniques improved, it was found that the method of inheritance of human characteristics is the same as that for other living things.

Some human geneticists, called genetic counsellors, advise individuals concerning the probabilities for the appearance of serious hereditary defects in their children. The counsellors usually have medical training because many traits are recognizable only after special diagnostic procedures. Medical genetics is another important application of human genetics. Many medical schools devote entire departments to medical genetics, which is the study of the treatment and prevention of inherited afflictions in man.

It is possible that man may someday control his heredity; even now functional genes can be transferred from one organism to another, and certain treatments are able to cause specific kinds of mutations. Such manipulation of genes eventually may be useful in solving many human hereditary diseases; e.g., stopping the function of genes that are out of control, starting the function of nonfunctioning ones. Activation of nonfunctioning genes in some types of tissue may enable them to replace body parts that have been injured or destroyed. It is conceivable that man may someday learn how to change harmful genes into normal ones.

Methods in genetics
Experimental breeding

When animals that differ with respect to one primary trait are bred, and their offspring then are bred among themselves to give a second generation, the method of inheritance of the trait can be determined; the process is known as a monohybrid cross. A dihybrid cross involves breeding individuals that differ with respect to two traits; the results of such crosses show whether the genes are linked on the same chromosome or are on different chromosomes. If the genes are linked, the distance between them can be determined by the number of recombinations of traits obtained, an indication of the amount of crossing over between genes. By such crosses, geneticists have established elaborate chromosome maps of many organisms showing the location of many genes on the chromosomes.

A test cross may be used to determine if animals carry recessive genes; e.g., cocker spaniel dogs may be of solid colours or parti-coloured (spotted). Since the gene for parti-coloured is recessive, it may be carried (but not expressed) by some solid-coloured dogs. If a solid-coloured dog is suspected of carrying a recessive gene for parti-coloured, it is bred to a parti-coloured dog. Parti-coloured offspring indicate that the solid-coloured dog carries the recessive gene. This technique is used by animal breeders to eliminate undesirable recessive genes.

Experimental breeding is most successful in organisms with large numbers of offspring, a relatively short life cycle, and a number of variable characteristics. The fruit fly, Drosophila, meets these requirements and has been used extensively in breeding experiments; mice also have been used extensively.

Cytogenetic techniques




Methods in genetics
Experimental breeding


Cytogenetic techniques

Cytogenetics focuses on the microscopic examination of genetic components of the cell, including chromosomes, genes, and gene products. Older cytogenetic techniques involve placing cells in paraffin wax, slicing thin sections, and preparing them for microscopic study. The newer and faster squash technique involves squashing entire cells and studying their chromosomescontents. Dyes that selectively stain various parts of the cell are used; the genes, for example, may be located by selectively staining the DNA of which they are composed. Radioactive compounds also and fluorescent tags are valuable in determining the location of various components of genes and gene products in the cell. Tissue-culture techniques may be used to grow cells before squashing; white blood cells can be grown from samples of human blood and studied with the squash technique. One major application of cytogenetics in humans is in diagnosing abnormal chromosomal complements such as Down syndrome (caused by an extra copy of chromosome 21) and Klinefelter syndrome (occuring in males with an extra X chromosome). Some diagnosis is prenatal, performed on cell samples from amniotic fluid or the placenta.

Biochemical techniques

Biochemistry is carried out at the cellular or subcellular level, generally on cell extracts. Biochemical methods are applied to the main chemical compounds of genetics—notably DNA, RNA, and protein. Biochemical techniques are used to determine the activities of genes within cells . Radioactive compounds are valuable in studies involving gene duplication and cell metabolism. Thymine and to analyze substrates and products of gene-controlled reactions. In one approach, cells are ground up and the substituent chemicals are fractionated for further analysis. Special techniques (e.g., chromatography and electrophoresis) are used to separate the components of proteins so that inherited differences in their structures can be revealed. For example, more than 100 different kinds of human hemoglobin molecules have been identified. Radioactively tagged compounds are valuable in studying the biochemistry of whole cells. For example, thymine is a compound found only in genesDNA; if radioactive thymine is placed in a tissue-culture medium in which cells are growing, genes use it to duplicate themselves. When cells containing radioactive thymine are analyzed, the results show that, during duplication, genes split the DNA molecule splits in half, and each half synthesizes its missing components.

When radioactive uracil, a compound found only in the ribonucleic acid (RNA) component of cells, is incorporated into the RNA messengers of genes, their pathway from the chromosomes to the site of protein synthesis in the cytoplasm (ribosomes) is revealed.Chemical tests are used to distinguish certain inherited characteristics conditions of manhumans; e.g., urinalysis and blood analysis reveal the presence of certain inherited abnormalities—phenylketonuria (PKU), cystinuria, alkaptonuria, gout, and galactosemia. Special techniques (e.g., chromatography, electrophoresis) are used to separate the components of proteins, so that inherited differences in their structures can be revealed; for example, more than 100 different kinds of human hemoglobin molecules have been identifiedGenomics has provided a battery of diagnostic tests that can be carried out on an individual’s DNA. Some of these tests can be applied to fetuses in utero.

Physiological techniques

Physiological techniques also are , directed at exploring functional properties or organisms, are also used in genetic investigations. In microorganisms, most genetic variations involve some important cell function. Some strains of one bacterium (Escherichia coli), for example, are able to synthesize the vitamin thiamine thiamin from simple compounds; others, which lack an enzyme necessary for this synthesis, cannot survive unless thiamine thiamin is already present. The two strains can be distinguished by placing them on a thiaminethiamin-free mixture; : those that grow have the gene for the enzyme, those that fail to grow do not. The technique also is applied to human cells, since many inherited human abnormalities are caused by a faulty gene that fails to produce a vital enzyme; albinism, which results from an inability to produce the pigment melanin in the skin, hair, or iris of the eyes, is an example of an enzyme deficiency in man.

Molecular techniques


Immunological techniques


Immunological techniques are used in the blood - group determinations that precede in blood transfusions, in organ transplants, and in determining Rh Rhesus incompatibility in childbirth.

Evolutionary relationships can be determined by immunological techniques. If protein from a fruit fly is injected into a guinea pig, and the guinea pig produces antibodies and its blood serum is then mixed with proteins from the fly, antigens and antibodies react to produce a cloudy mixture. Mixtures of guinea pig blood serum and proteins from other fruit-fly species cause various degrees of cloudiness, depending on their evolutionary relationship to the original species; e.g., the closer the relationship, the greater degree of cloudiness.

Mathematical techniques

Mathematical Specific antigens of the human leukocyte antigen (HLA) genes are correlated with human diseases and disease predispositions. Antibodies also have a genetic basis, and their seemingly endless ability to match any antigen presented is based on special types of DNA shuffling processes between antibody genes. Immunology is also useful in identifying specific recombinant DNA clones that synthesize a specific protein of interest.

Mathematical techniques

Because much of genetics is based on quantitative data, mathematical techniques are used extensively in genetics. The laws of probability are applicable to crossbreeding and are used to predict ratios concerning the appearance frequencies of specific traits genetic constitutions in offspring. Geneticists also use statistical methods to determine the significance of deviations from expected results in experimental analyses. In investigations involving possible mutagenic effects of factors such as high-energy radiation and drugs, statistical tests are used to establish the validity of conclusions; statistics are used in studies of the possible effects of LSD in producing chromosome aberrations in man, for example, to show whether differences found in cells of users and nonusers of the drug are significant.Mathematics is used by population geneticists to evaluate the distribution of genes in populations. The Hardy–Weinberg principle, for example, is important in studying animals that carry a recessive gene; when the actual number of carriers is much greater than that calculated, it is concluded that some environmental factor favours the carriers. The gene for sickle-cell anemia in black Africans, for instance, is found in more people than the frequency of those who have the anemia would indicate because people who carry the gene are more resistant to malaria than noncarriers and, therefore, have a better chance of survivaladdition, population genetics is based largely on mathematical logic—for example, the Hardy-Weinberg equilibrium and its derivatives (see above).


Applied genetics

Genetic techniques are used in medicine to diagnose and treat inherited human disorders. Knowledge of a family history of conditions such as cancer or tuberculosis various disorders may indicate a hereditary tendency to develop these afflictions. Cells from embryonic membranes tissues reveal certain genetic abnormalities, including enzyme deficiencies, that may be present in newborn babies, and thus permit permitting early treatment. Many countries require a blood test of newborn babies to determine the presence of an enzyme necessary to convert an amino acid, phenylalanine, into simpler products. Phenylketonuria (PKU), which results from lack of the enzyme, causes permanent brain damage if not treated soon after birth. The presence of approximately 100 Many different types of human genetic diseases can be detected in embryos as young as 12 weeks; the procedure , called amniocentesis, involves removal and testing of a small amount of fluid from around the embryo (called amniocentesis) or of tissue from the placenta (called chorionic villus sampling).


Agriculture and animal husbandry


Plant geneticists use special techniques to produce new species by special treatment; , such as hybrid grains (i.e.g., a hybrid grain has been produced from produced by crossing wheat and rye), and plants resistant to destruction by insect and fungal pests have been developed.

Plant breeders use the techniques of budding and grafting to maintain desirable gene combinations originally obtained from crossbreeding. Transgenic plant cells can be made into plants by growing the cells on special hormones. The use of the chemical compound colchicine, which causes chromosomes to double in number, has resulted in many new varieties of fruits, vegetables, and flowers. Animal breeders use artificial insemination to propagate the genes of prize bulls. Prize cows can transmit their genes to hundreds of offspring by hormone treatment, which stimulates the release of many eggs that are collected, fertilized, and transplanted to foster mothersMany transgenic lines of crop plants are commercially advantageous and are being introduced into the market.


Various industries employ geneticists; the brewing industry, for example, may use geneticists to obtain improve the strains of yeast that produce large quantities of alcohol. The pharmaceutical industry has developed strains of molds, bacteria, and other microorganisms high in antibiotic yield.

David A. Micklos and Greg A. Freyer, DNA Science: A First Course in Recombinant DNA Technology (1990), provides an introductory guide that covers well the history, concepts, and applications of molecular biology. Paul Berg and Maxine Singer, Dealing with Genes: The Language of Heredity (1992), is a well-illustrated overview of molecular genetics and its relationship with developmental biology, medicine, and biochemistry. Anthony J.F. Griffiths et al., An Introduction to Genetic Analysis, 5th ed. (1993); and Benjamin Lewin, Genes V (1994), emphasize the molecular aspects of genetics, although the former work deals more extensively with population genetics, quantitative genetics, and evolution. A complementary work is Roger L.P. Adams, DNA Replication (1991). R.W. Old and S.B. Primrose, Principles of Gene Manipulation: An Introduction to Genetic Engineering, 5th ed. (1994), is one of the best texts dealing exclusively with genetic engineering. James D. Watson et al., Recombinant DNA, 2nd ed. (1992), provides a relatively short but extremely useful treatment of the methods and applications of recombinant DNA technology and other current methods in molecular genetics; to some extent it updates an earlier work, James D. Watson et al., Molecular Biology of the Gene, 4th ed., 2 vol. (1987), which still retains utility despite having been overtaken by the sheer momentum of theory and technology in this subject.

Gunther S. Stent and Richard Calendar, Molecular Genetics: An Introductory Narrative, 2nd ed. (1978), although out of date in many parts, nonetheless provides a history of genetics beginning with medieval observations on inheritance and traces progress to the late 1970s. S. Brenner (compiler), Molecular Biology: A Selection of Papers (1989), collects a series of the most important research publications on molecular genetics appearing in the Journal of Molecular Biology from the 1950s onward. The best historical surveys of molecular biology are Horace Freeland Judson, The Eighth Day of Creation: Makers of the Revolution in Biology (1979), which concentrates on research after World War II; and Robert Olby, The Path to the Double Helix (1974, reissued 1994), which focuses on research from the 1920s to the 1950s. For the same period, Robert E. Kohler, Lords of the Fly: Drosophila Genetics and the Experimental Life (1994), studies the research and culture of early Drosophila geneticists; while Lily E. Kay, The Molecular Vision of Life (1993), examines the funding and politics behind the rise of molecular biology.



Historical accounts

Nina Fedoroff and David Botstein (eds.), The Dynamic Genome: Barbara McClintock’s Ideas in the Century of Genetics (1992), treats this scientist’s work and the history of modern genetics.

McClintock’s research is also ably described in Evelyn Fox Keller, A Feeling for the Organism (1983, reissued 1993). Zhores A. Medvedev, The Rise and Fall of T.D. Lysenko (1969), documents a particularly negative chapter in the history of Soviet genetic study, the promulgation of the idea of the inheritance of acquired characters.




the discoverers. Anne Sayre, Rosalind Franklin and DNA (1975); and Francis Crick, What Mad Pursuit (1988), give somewhat different accounts of the discovery of DNA structure. Works delineating the science and sociology of the discovery are John Cairns, Gunther S. Stent, and James D. Watson (eds.), Phage and the Origins of Molecular Biology, expanded ed. (1992); and Maclyn McCarty, The Transforming Principle: Discovering That Genes Are Made of DNA (1985).


, the polymerase chain reaction.

Bernadette Modell and Michael Modell, Towards a Healthy Baby: Congenital Disorders and the New Genetics in Primary Care (1992), summarizes knowledge about many common genetic disorders in humans and the health approaches and policies regarding their diagnosis and treatment. Richard Dawkins, The Selfish Gene, rev. ed. (1989), is an easily read book espousing the author’s views on evolution.


Genetics texts


Bacterial genetics texts

The rise of contemporary molecular genetics

, which now extends to all known cell types,

had its inception in studies on bacterial genetics and

, in particular,

on viruses that infect bacteria.

Those foundational studies have made bacteria the best-known organisms from a genetic point of view. Arnold J. Levine, Viruses (1992), is an excellent treatment. Fundamental ideas about

Molecular genetics related to bacterial structure and function

from a molecular genetic viewpoint

are presented in

Thomas D. Brock, The Emergence of Bacterial Genetics (1990), a detailed perspective; T.A. Brown (ed.), Essential Molecular Biology: A Practical Approach, 2 vol. (1991), much of which is concerned with the genetics of bacteria; Stephen Cooper, Bacterial Growth and Division: Biochemistry and Regulation of Prokaryotic and Eukaryotic Division Cycles (1991), an important contribution; John L. Ingraham, Ole Maaløe, and

Frederick C. Neidhardt,

Growth of the Bacterial Cell (1983); and Frederick C. Neidhardt,

John L. Ingraham, and Moselio

