Hardy–Weinberg Equilibrium

The Hardy-Weinberg (HW) principle is a fundamental theorem of evolutionary genetics stating that, under random mating and Mendelian inheritance, the genotype frequencies can be calculated from the allele frequencies and that both genotype and allele frequencies remain stable in the absence of any perturbing forces. Most commonly, the HW-principle is used to test whether the observed genotype frequencies in a sample of individuals match the expected frequencies. For a biallelic locus with alleles A and a that have frequencies p and q, the expected frequencies of the three possible genotypes AA, Aa, and aa are p², 2pq, and q², respectively. However, the HW-principle depends on a number of assumptions that may not be met in real populations: random mating, infinite population size, no mutation, no selection, a single population, no migration, non-overlapping generations, and diploid inheritance. Many tests have been developed for testing the deviation from HW expectations for a sample of genotypes; of these tests, the exact test is the method of choice for single loci. Testing such deviation has many applications in population genetic studies, not only for weeding out loci that suffer from genotyping errors, but also for testing ecological or evolutionary hypotheses regarding demography and mating systems.