Hardy–Weinberg principle

The Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include mate choice, mutation, selection, genetic drift, gene flow and meiotic drive. Because one or more of these influences are typically present in real populations, the Hardy–Weinberg principle describes an ideal condition against which the effects of these influences can be analyzed.

In the simplest case of a single locus with two alleles denoted A and a with frequencies f(A) = p and f(a) = q, respectively, the expected genotype frequencies are f(AA) = p² for the AA homozygotes, f(aa) = q² for the aa homozygotes, and f(Aa) = 2pq for the heterozygotes. The genotype proportions p², 2pq, and q² are called the Hardy–Weinberg proportions. Note that the sum of all genotype frequencies of this case is the binomial expansion of the square of the sum of p and q, and such a sum, as it represents the total of all possibilities, must be equal to 1. Therefore, (p + q)² = p² + 2pq + q² = 1. The realistic solution of this equation is q = 1 − p.

If union of gametes to produce the next generation is random, it can be shown that the new frequency f′ satisfies ${\displaystyle \textstyle f'({\text{A}})=f({\text{A}})}$ and ${\displaystyle \textstyle f'({\text{a}})=f({\text{a}})}$. That is, allele frequencies are constant between generations, so equilibrium is reached.

...
Wikipedia