7.5 Hardy-Weinberg Equilibrium

A 'not evolving' baseline

Hardy-Weinberg equilibrium is a null model: allele frequencies stay constant when a population is not evolving. It holds only if five conditions are met: no mutation, random mating, no gene flow, a very large population, and no natural selection. Deviations from it signal that evolution is happening.

The equations

Using p (dominant allele frequency) and q (recessive): p + q = 1. Genotype frequencies follow p² + 2pq + q² = 1, where p² = homozygous dominant, 2pq = heterozygous, and q² = homozygous recessive. These let you calculate expected frequencies.

Key terms

Hardy-Weinberg equilibrium

A model where allele frequencies stay constant because the population is not evolving.

p + q = 1

The frequencies of the two alleles must add up to 1.

p² + 2pq + q² = 1

Genotype frequencies: p² (hom. dom.) + 2pq (het.) + q² (hom. rec.) = 1.

Five conditions

No mutation, random mating, no gene flow, large population, no selection.

2pq = heterozygotes

The 2pq term gives the frequency of heterozygous individuals.

Try an AP-style question

What is the frequency of the dominant allele (p)?

1. 0.84
2. 0.16
3. 0.6
4. 0.4