What equation describes genotype frequencies in an ideal non-evolving population?

The Hardy–Weinberg equation describes genotype frequencies in an ideal non-evolving population. Its standard form is p² + 2pq + q² = 1, where p and q are the frequencies of two alleles.

The principle assumes a very large population, random mating, no mutation, no migration, and no natural selection. Under those conditions, allele and genotype frequencies remain constant from one generation to the next. The equation provides a baseline against which real populations can be compared.

A population does not need to satisfy every assumption perfectly for the equation to be useful, but departures can change the expected frequencies. Hardy–Weinberg calculations are often used to estimate carrier frequencies for recessive alleles. The principle describes a model of genetic stability; it is not a claim that real populations never evolve.