Hardy–Weinberg equilibrium

Hardy–Weinberg equilibrium is the population-genetics equivalent of a level table: if nothing pushes on it, it stays put. It describes an idealized population in which allele and genotype frequencies do not change from generation to generation, because none of the forces of evolution are acting.

Under random mating, with alleles A and a at frequencies p and q (where p + q = 1), the expected genotype frequencies are p² for AA, 2pq for Aa, and q² for aa. These three add to one. The result holds only under several assumptions: no mutation, no migration, no selection, infinitely large population (so no drift), and random mating.

Its real power is as a null model. Because the conditions are never perfectly met in nature, deviations from the expected proportions are informative. If observed genotype frequencies stray from p², 2pq, q², something on the list is at work—and that something is usually what a geneticist wants to find.

The principle, derived independently in 1908 by Godfrey Hardy and Wilhelm Weinberg, also reassures us that dominant alleles do not automatically increase in frequency. Heredity alone shuffles the cards; it does not change how many of each there are.