random mating

Meaning: In random mating, mate choice is unrelated to the trait or gene being considered, so the chance of a given pairing depends only on how common each type is. Nonrandom mating includes assortative mating (choosing similar partners, such as by height or education), disassortative mating (choosing dissimilar partners), inbreeding, and mate choice driven by sexual selection. Assortative mating and inbreeding raise the share of homozygotes. Nonrandom mating changes genotype frequencies but does not by itself change allele frequencies. Natural selection, mutation, migration, and genetic drift do that.

Equation: p² + 2pq + q² = 1, with p + q = 1. Here p and q are the frequencies of two alleles, and p², 2pq, and q² are the expected genotype frequencies under random mating. All are unitless fractions.

On the MCAT: A passage lists the Hardy-Weinberg conditions (large population, no mutation, no migration, no selection, random mating). You are asked which condition a scenario violates. Peahens preferring males with long tails is nonrandom mating through sexual selection. Partners choosing each other for similar traits is assortative mating.

Common confusion: Students think nonrandom mating changes allele frequencies. It changes how alleles are combined into genotypes, not how common the alleles are. Mating can also be nonrandom for one trait yet random for an unrelated gene.