HWE

Hardy-Weinberg Equilibrium (HWE) is a fundamental principle in population genetics describing how allele and genotype frequencies remain constant from generation to generation in an ideal population. It provides a null model for understanding evolutionary processes and for interpreting genetic data.

The equilibrium applies to large, randomly mating populations with no mutation, migration, or natural selection. Under these conditions, allele frequencies p and q (with p + q = 1) determine genotype frequencies as p^2 for the homozygous dominant, 2pq for the heterozygous, and q^2 for the homozygous recessive genotypes. These proportions remain constant across generations if the assumptions hold, even as individuals reproduce.

HWE is used in both theoretical and applied genetics. Practically, researchers compare observed genotype counts in a sample to those expected under HWE (calculated from allele frequencies) using a chi-squared test. Deviations can indicate factors such as nonrandom mating, natural selection, population structure, genetic drift, mutation, or technical issues like genotyping errors. In genome-wide association studies, testing for HWE serves as a quality control step to flag problematic data.

History and caveats: HWE was independently proposed by Godfrey Harold Hardy and Wilhelm Weinberg in 1908. It represents an idealized baseline; most natural populations deviate due to evolutionary forces. Extensions exist for special cases, including sex-linked loci, where allele transmission differs between males and females, and for small or structured populations where drift and migration alter expectations.