allele frequency

Picture a giant jar holding every copy of a particular gene in a population—two copies from each person, since we are diploid. If 30 out of every 100 copies in the jar are the version we call A, then A's allele frequency is 0.30, or 30%. It is simply the share of the pie taken by one variant.

More precisely, allele frequency is the proportion of all alleles at a given locus that are of a particular type. The frequencies of all alleles at that locus must add up to one. For a locus with two alleles, the two frequencies are traditionally written p and q, with p + q = 1.

Allele frequency is the central quantity of population genetics. Evolution, defined genetically, is change in allele frequency from one generation to the next. Mutation creates new alleles, while drift, gene flow, and selection raise or lower the frequencies of existing ones.

Frequencies are always relative to a chosen population at a chosen time. The same allele can be common in one group and vanishingly rare in another, which is why context matters when a frequency is quoted.