The gene is defined as the unit of inheritance. A gene is actually a sequence of DNA (see nucleic acid) contained by and arranged linearly along a chromosome. Each gene transmits chemical information that is expressed as a trait, e.g., tall or dwarf size in the garden pea plant. Each species has a genome, or characteristic set of genes, that contains the total genetic information for an individual organism. In many familiar organisms two genes for each trait are present in each individual, and these paired genes, both governing the same trait, are called alleles. The two allelic genes in any one individual may be alike (homozygous) or different (heterozygous). The chromosomes of animals and plants that reproduce sexually usually exist in pairs; the members of a chromosome pair are termed homologous (see reproduction). In humans there are 46 chromosomes, or 23 homologous pairs. Pairs of genes are borne on homologous chromosomes.

In the process of meiosis, by which ova and sperm are produced, the chromosomes are so divided that each mature sex cell contains half the original number of chromosomes, or one chromosome of each pair, and therefore one gene of each pair. Thus, when the ovum and the sperm fuse on fertilization, the fertilized egg (zygote) receives one allele from each parent. With many pairs of alleles that have contrasting effects (e.g., certain alleles produce different eye color), one is dominant and the other recessive: an individual heterozygous (carrying contrasting alleles) for a given characteristic invariably displays one aspect of that characteristic and not its alternative, although the gene for the aspect that does not appear (i.e., that is recessive) is present. This individual is called a hybrid.

In Mendelian law (see Mendel) the offspring—or first filial (called F1) generation—of parents each homozygous for different alleles of a given gene are all hybrids heterozygous for the characteristic governed by that gene and are said to be of the same phenotype, i.e., they are all similar in appearance to the homozygous dominant parent because the recessive characteristic is masked, although their gene composition, or genotype, is different from either parent. A cross of members of the F1 generation produces a second filial (F2) generation of which approximately three fourths show the dominant characteristic and one fourth the recessive. Note however, that great numbers of characteristics are inherited simultaneously and the patterns of transmission of genes are such that offspring strongly resembling one parent in certain traits can resemble the other parent in other traits.

It has also become clear that an individual organism's heredity and environment interact in the manifestation of many traits: a pea plant with a genetic tendency toward tallness will not achieve its full size if deprived of adequate water and minerals for growth. However, true alterations in gene and chromosome structure are the product of mutation and are not produced by environmental conditions, as was postulated by the theory of acquired characteristics. The discovery by H. J. Muller in 1927 of methods for artificially inducing mutations by means of ionizing radiations and other mutagens opened the way for much new genetics research.

Modification of Mendel's principles developed as knowledge of the chromosomes increased; many discoveries have helped to account for apparent deviations from Mendelian ratios. For example, Mendel's studies emphasized genes that behave independently from one another during transmission to offspring. But we now know that genes are transmitted as constituents of chromosomes, each of which carries many different genes, which sheds light on the tendency of certain characteristics to appear in combination with one another (linkage). It also has been found that some characteristics are sex-linked, i.e., are transmitted by genes carried by the sex chromosomes (see sex); and that a non-sex-linked gene inherited from the father may differ in its expression from the same gene inherited from the mother, a phenomenon called imprinting. Other research has shown that there may be multiple alleles (more than two alternative genes) for a given characteristic: the human blood groups are determined by a combination of several possible alleles. It is apparent that homologous portions of paired chromosomes may be interchanged during meiosis (crossing over) and that the interaction of many genes is responsible for determining many of the traits of individuals. Since the discovery (1953) of the structure of DNA, work on nucleic acids has begun to explain how genes determine life processes by directing the synthesis of proteins. It has also explained mutations as alterations in gene or chromosome structure. It has been found, for example, that mutations in the form of repeated sequences of otherwise normal chemical bases, can grow in length with succeeding generations, in some cases causing diseases (e.g., myotonic muscular dystrophy) that increase in severity each time they are inherited.

Most of the knowledge of chromosome structure and the behavior of genes has come from studies of the vinegar, or fruit, fly (Drosophila melanogaster), which reproduces so rapidly that many generations can be studied over a short time. The work of T. H. Morgan and his associates on Drosophila was the basis of much of the early progress of genetics in the United States. Certain other small laboratory animals, plants, and microorganisms such as the E. coli bacteria are now used, also largely because of their ability to reproduce rapidly. For obvious reasons human beings are poor subjects for experimental genetic studies; however, much that aids understanding heredity in humans has been learned from the lower forms of life. Also, by tracing the appearance of certain abnormal characteristics (e.g., hemophilia, color blindness, and certain mental disorders and anatomical defects) and blood groups through a number of generations the hereditary pattern of these conditions has been established. The increasing ability of scientists to decode genetic information (see Human Genome Project) has led to a considerable expansion of knowledge about the nature and role of genes in humans and other organisms. Application of this knowledge has played an important role in the fields of gene therapy, genetic engineering, and evolutionary studies, and has resulted in a better understanding of the genetic components of disease, physical characteristics, mental illness, and even personality.

The study of mutations, together with the analyses of population genetics, has been used to explain the mechanism of evolution. The elementary process of evolution is considered to be the changes in the frequency of occurrence of alleles in a population. Mutation, which causes the appearance of new alleles or changes the relative frequency of already existing alleles, is one important mechanism by which evolution occurs. Natural selection (see selection), by affecting reproductive success, influences the frequencies of alleles and other genetic variants in successive generations. For example, if the presence of a particular allele makes a homozygous individual unable to mate, the allele may be eliminated from the population.

Genetic drift —the random fluctuation in the frequency of an allele, resulting mainly from the vagaries of chance mating—is also an evolutionary mechanism. Although in large populations drift varies only a little above and below a statistical mean, in small breeding populations an entire generation might, by chance alone, be born with the same genotype with respect to a particular allelic pair of genes, thus leading to either the elimination or dominance of a particular gene. Because fluctuations in the proportions of alleles are more significant in the gene pools of small, isolated breeding populations, genetic drift is a mechanism of species diversity and evolution in such groups.