Nonrandom Mating Alters Genotype Frequencies

An assumption of the Hardy–Weinberg law is that mating is random with respect to genotype. Although it does not alter the frequencies of alleles, nonrandom mating affects the way in which alleles combine to form genotypes and alters the genotypic frequencies of a population.

One form of nonrandom mating is inbreeding, which is preferential mating between related individuals. Inbreeding causes a departure from the Hardy–Weinberg equilibrium frequencies of p2, 2pq, and q2. More specifically, it leads to an increase in the proportion of homozygotes and a decrease in the proportion of heterozygotes in a population. Close inbreeding is often harmful because it increases the proportion of homozygotes and thereby boosts the probability that deleterious and lethal recessive alleles will combine to produce homozygotes with a harmful trait. This increased appearance of lethal and deleterious traits with inbreeding is termed inbreeding depression.

CONCEPTS

Nonrandom mating alters the frequencies of genotypes but not the frequencies of alleles. Inbreeding is preferential mating between related individuals. With inbreeding, the frequency of homozygotes increases, whereas the frequency of heterozygotes decreases.