Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle describes what would happen to a population's allele and genotype frequencies if it were NOT evolving - it is a null model, analogous to a control in an experiment. If a real population's genotype frequencies match Hardy-Weinberg predictions generation after generation, that population is, for the gene in question, in genetic equilibrium and not evolving.

Allele frequencies: p + q = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele in the gene pool. Genotype frequencies: p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant individuals, 2pq is the frequency of heterozygotes, and q² is the frequency of homozygous recessive individuals.

Because the recessive phenotype directly reveals the homozygous recessive genotype (q²), it is usually the easiest starting point in a calculation: take the square root of the recessive phenotype's frequency to find q, then p = 1 − q, and use these to find every other frequency in the population.

In practice, virtually no real population perfectly satisfies all five assumptions, which is precisely why real populations evolve. When observed genotype frequencies in a sampled population differ from Hardy-Weinberg-predicted frequencies, this signals that evolution is occurring and that one or more assumptions are being violated. The next analytical step - a hallmark of AP free-response questions - is identifying which mechanism(s) (selection, drift, gene flow, non-random mating, mutation) most plausibly explains the observed deviation given the scenario described.