The Hardy-Weinberg ratio is the starting point for much of the theory of population genetics. It is the ratio of genotype frequencies that evolve when mating is random and neither selection nor drift are operating. For two alleles (A and a) with frequencies p and q, the Hardy-Weinberg frequencies are
Genotype AA : Aa : aa
Frequency p2 : 2pq : q2
These frequencies are reached after a single generation of random mating from any initial genotype frequencies. The Hardy-Weinberg ratio can be understood in terms of simple combinatorial probability.