introduction to genetics introduction to genetics genetics is the study of genes and tries to explain what they are and how they work. genes are how living organisms inherit features or traits from their ancestors; for example, children usually look like their parents because they have inherited their parents' genes. genetics tries to identify which traits are inherited and to explain how these traits are passed from generation to generation. this article is a non-technical introduction to the subject. for the main encyclopedia article, see genetics. part of a series ongenetics key components chromosome dna rna genome heredity mutation nucleotide variation outline index history and topics introduction history evolution (molecular) population genetics mendelian inheritance quantitative genetics molecular genetics research geneticist dna sequencing genetic engineering genomics ( template) medical genetics branches of genetics personalized medicine personalized medicine category genetics glossary dnaa long molecule that looks like a twisted ladder. it is made of four types of simple units and the sequence of these units carries information, just as the sequence of letters carries information on a page. nucleotidethey form the rungs of the dna ladder and are the repeating units in dna. there are four types of nucleotides (a, t, g and c) and it is the sequence of these nucleotides that carries information. chromosomea package for carrying dna in the cells. they contain a single long piece of dna that is wound up and bunched together into a compact structure. different species of plants and animals have different numbers and sizes of chromosomes. genea segment of dna. genes are like sentences made of the "letters" of the nucleotide alphabet, between them genes direct the physical development and behavior of an organism. genes are like a recipe or instruction book, providing information that an organism needs so it can build or do something - like making an eye or a leg, or repairing a wound. allelethe different forms of a given gene that an organism may possess. for example, in humans, one allele of the eye-color gene produces green eyes and another allele of the eye-color gene produces brown eyes. genomethe complete set of genes in a particular organism. genetic engineeringwhen people change an organism by adding new genes, or deleting genes from its genome. mutationan event that changes the sequence of the dna in a gene. some traits are part of an organism's physical appearance, such as a person's eye color, height or weight. other sorts of traits are not easily seen and include blood types or resistance to diseases. some traits are inherited through our genes, so tall and thin people tend to have tall and thin children. other traits come from interactions between our genes and the environment, so a child might inherit the tendency to be tall, but if they are poorly nourished, they will still be short. the way our genes and environment interact to produce a trait can be complicated. for example, the chances of somebody dying of cancer or heart disease seems to depend on both their genes and their lifestyle. genes are made from a long molecule called dna, which is copied and inherited across generations. dna is made of simple units that line up in a particular order within this large molecule. the order of these units carries genetic information, similar to how the order of letters on a page carries information. the language used by dna is called the genetic code, which lets organisms read the information in the genes. this information is the instructions for constructing and operating a living organism. the information within a particular gene is not always exactly the same between one organism and another, so different copies of a gene do not always give exactly the same instructions. each unique form of a single gene is called an allele. as an example, one allele for the gene for hair color could instruct the body to produce much pigment, producing black hair, while a different allele of the same gene might give garbled instructions that fail to produce any pigment, giving white hair. mutations are random changes in genes and can create new alleles. mutations can also produce new traits, such as when mutations to an allele for black hair produce a new allele for white hair. this appearance of new traits is important in evolution. genes and inheritance a section of dna; the sequence of the plate-like units (nucleotides) in the center carries information. genes are pieces of dna that contain information for the synthesis of ribonucleic acids (rnas) or polypeptides. genes are inherited as units, with two parents dividing out copies of their genes to their offspring. humans have two copies of each of their genes, but each egg or sperm cell only gets one of those copies for each gene. an egg and sperm join to form a complete set of genes. the resulting offspring has the same number of genes as their parents, but for any gene, one of their two copies comes from their father, and one from their mother. the effects of this mixing depend on the types (the alleles) of the gene. if the father has two copies of an allele for red hair, and the mother has two copies for brown hair, all their children get the two alleles that give different instructions, one for red hair and one for brown. the hair color of these children depends on how these alleles work together. if one allele dominates the instructions from another, it is called the dominant allele, and the allele that is overridden is called the recessive allele. in the case of a daughter with alleles for both red and brown hair, brown is dominant and she ends up with brown hair. a punnett square showing how two brown haired parents can have red or brown haired children. 'b' is for brown and 'b' is for red. red hair is a recessive trait. although the red color allele is still there in this brown-haired girl, it doesn't show. this is a difference between what you see on the surface (the traits of an organism, called its phenotype) and the genes within the organism (its genotype). in this example you can call the allele for brown "b" and the allele for red "b". (it is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) the brown hair daughter has the "brown hair phenotype" but her genotype is bb, with one copy of the b allele, and one of the b allele. now imagine that this woman grows up and has children with a brown-haired man who also has a bb genotype. her eggs will be a mixture of two types, one sort containing the b allele, and one sort the b allele. similarly, her partner will produce a mix of two types of sperm containing one or the other of these two alleles. when the transmitted genes are joined up in their offspring, these children have a chance of getting either brown or red hair, since they could get a genotype of bb = brown hair, bb = brown hair or bb = red hair. in this generation, there is, therefore, a chance of the recessive allele showing itself in the phenotype of the children—some of them may have red hair like their grandfather. many traits are inherited in a more complicated way than the example above. this can happen when there are several genes involved, each contributing a small part to the result. tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. however, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or red hair. this is because of the large number of genes involved; this makes the trait very variable and people are of many different heights. despite a common misconception, the green/blue eye traits are also inherited in this complex inheritance model. inheritance can also be complicated when the trait depends on the interaction between genetics and environment. for example, malnutrition does not change traits like eye color, but can stunt growth. how genes work genes make proteins the function of genes is to provide the information needed to make molecules called proteins in cells. cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like bacteria are just a single cell. a cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells divide. there is a simple division of labor in cells—genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing the damage. each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. genes tell cells what to do by telling them which proteins to make and in what amounts. genes are expressed by being transcribed into rna, and this rna then translated into protein. proteins are made of a chain of 20 different types of amino acid molecules. this chain folds up into a compact shape, rather like an untidy ball of string. the shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein does. for example, some proteins have parts of their surface that perfectly match the shape of another molecule, allowing the protein to bind to this molecule very tightly. other proteins are enzymes, which are like tiny machines that alter other molecules. the information in dna is held in the sequence of the repeating units along the dna chain. these units are four types of nucleotides (a,t,g and c) and the sequence of nucleotides stores information in an alphabet called the genetic code. when a gene is read by a cell the dna sequence is copied into a very similar molecule called rna (this process is called transcription). transcription is controlled by other dna sequences (such as promoters), which show a cell where genes are, and control how often they are copied. the rna copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the rna into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. the new protein then folds up into its active form. the process of moving information from the language of rna into the language of amino acids is called translation. dna replication. dna is unwound and nucleotides are matched to make two new strands. if the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change—if part of a gene is deleted, the protein produced is shorter and may not work anymore. this is the reason why different alleles of a gene can have different effects on an organism. as an example, hair color depends on how much of a dark substance called melanin is put into the hair as it grows. if a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. however, if the alleles for a particular protein have different sequences and produce proteins that can't do their jobs, no melanin is produced and the person has white skin and hair (albinism). genes are copied genes are copied each time a cell divides into two new cells. the process that copies dna is called dna replication. it is through a similar process that a child inherits genes from its parents when a copy from the mother is mixed with a copy from the father. dna can be copied very easily and accurately because each piece of dna can direct the assembly of a new copy of its