p + q = 1 | p² + 2pq + q² = 1
The Hardy-Weinberg principle states that allele and genotype frequencies remain constant in a population across generations in the absence of evolutionary forces. For a two-allele system: p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies). Conditions: no mutation, no selection, random mating, large population, no migration. If 16% of a population shows the recessive phenotype (q² = 0.16), then q = 0.4, p = 0.6, carriers (2pq) = 48%. This principle is foundational in population genetics and medical genetics for estimating carrier frequencies of genetic disorders like cystic fibrosis (q² ≈ 1/2500 in Caucasians).