Hardy–Weinberg equilibrium (HWE) is the state of the genotypic frequency of two alleles of one autosomal gene locus after one discrete generation of random mating in an indefinitely large population: if the alleles are A and a with frequencies p and q(=1-p), then the equilibrium gene frequencies are simply p and q and the equilibrium genotypic frequencies for AA, Aa and aa are p2, 2pq and q2. It was independently identified in 1908 by G. H. Hardy and W. Weinberg after earlier attempts by W. E. Castle and K. Pearson. Weinberg, well known for pioneering studies of twins, made many important contributions to genetics, especially human genetics. Existence of this equilibrium provides a reference point against which the effects of selection, linkage, mutation, inbreeding and chance can be detected and estimated. Its discovery marked the initiation of population genetics.