Hardy-Weinberg principle
Gene frequencies in populations and the factors that change them.
The essentials
In a given population, one can find the frequency of occurrence of alleles of a gene. The Hardy–Weinberg principle states that allele frequencies in a population are stable and constant from generation to generation; the gene pool (total genes and their alleles in a population) remains constant. This is called genetic equilibrium.
If p is the frequency of allele A and q is the frequency of allele a in a diploid population, then p + q = 1. The frequencies of genotypes are given by the binomial expansion (p + q)² = p² + 2pq + q² = 1, where p² is the frequency of AA, 2pq of Aa and q² of aa.
When measured frequencies differ from the expected values, the difference (its direction) indicates the extent of evolutionary change. Disturbance of genetic equilibrium, that is, change in allele frequencies, is interpreted as evolution.
Key points
- Five factors affect Hardy–Weinberg equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection.
- Equilibrium assumes a large, randomly mating population with no mutation, migration or selection.
- To solve problems, start with the homozygous recessive frequency (q²), find q = √(q²), then p = 1 − q.
Exam tips
Always check your answer: p² + 2pq + q² must add up to 1. Convert percentages to decimals first.
Common mistakes
- Do not take the frequency of the recessive phenotype as q; it is q².
- The dominant phenotype frequency is p² + 2pq, not just p².