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.