Hardy–Weinberg equilibrium
For two alleles \(A\) and \(a\) with frequencies \(p\) and \(q=1-p\), random mating produces genotype frequencies
\[AA:p^2,\qquad Aa:2pq,\qquad aa:q^2.\]These sum to one because
\[(p+q)^2=p^2+2pq+q^2=1.\]Assumptions
The classical equilibrium assumes random mating and, for allele frequencies to remain unchanged, absence of evolutionary forces such as selection, mutation, migration and genetic drift.
Why it matters
Hardy–Weinberg proportions provide a baseline against which departures caused by biological processes can be studied.
Key idea. Hardy–Weinberg equilibrium is a reference model linking allele frequencies to expected genotype frequencies under idealised conditions.