allele frequency calculations

Meaning: Allele frequency is the fraction of all copies of a gene in a population that are a given allele. It can be found by counting: (2 × homozygotes for the allele + heterozygotes) ÷ 2N, where N is the number of individuals. Hardy-Weinberg equilibrium links allele frequencies to genotype frequencies. It requires a very large population, random mating, and no mutation, migration, or natural selection.

Equation: p + q = 1 and p² + 2pq + q² = 1. Here p and q are the frequencies of the dominant and recessive alleles, and p², 2pq, and q² are the frequencies of homozygous dominant, heterozygous, and homozygous recessive genotypes. All are unitless fractions.

On the MCAT: A passage gives the incidence of a recessive trait, such as 1 in 10,000. Then q² = 0.0001, q = 0.01, p = 0.99, and the carrier frequency 2pq ≈ 0.02. Other questions ask which violated assumption (selection, genetic drift, migration, nonrandom mating) would shift allele frequencies, or whether observed genotype counts match the expected ones.

Common confusion: q² is the frequency of the recessive genotype, not of the allele, so take the square root first. The dominant phenotype frequency is p² + 2pq, not p². When a recessive trait is rare, most copies of the recessive allele are carried by heterozygotes, because q is much larger than q².