Hardy-Weinberg

Meaning: The Hardy-Weinberg principle describes a population that is not evolving. If the population is very large, mates randomly, and has no mutation, no migration and no natural selection, allele frequencies stay constant across generations and genotype frequencies follow fixed proportions. A departure from those proportions signals that evolution is occurring at that locus.

Equation: p + q = 1 and p² + 2pq + q² = 1. Here p and q are the frequencies of the dominant and recessive alleles; p², 2pq and q² are the frequencies of the homozygous dominant, heterozygous and homozygous recessive genotypes. Each is a unitless fraction between 0 and 1.

On the MCAT: A passage gives the frequency of a recessive trait, such as 1 in 10,000. Then q² = 0.0001, q = 0.01, p = 0.99, and carriers (2pq) ≈ 0.02. Other questions ask which change, such as genetic drift in a small population, selection, migration or nonrandom mating, would move a population away from equilibrium.

Common confusion: The frequency of the recessive phenotype is q², not q, so take the square root first. Equilibrium also does not require p = q = 0.5; many pairs of values can be at equilibrium.