Allele frequency ($p_A$)

The allele frequency is the proportion of gene copies at a specified genetic locus carrying an allele. Two unrelated copies can have the same observed allele without being identical by descent; under independent sampling their matching probability is the sum of squared allele frequencies. Pedigree autozygosity instead traces copies to the same ancestral copy.

An allele fixes when all copies at the relevant locus in the population are that allele. Fixation can occur through selection or genetic drift. Spatial polymorphism means that fixation within an individual habitat need not imply fixation across a metapopulation.

The probability that an allele eventually occupies every copy at the specified genetic locus under a population model. For a rare favorable variant in a suitable large-population, small-effect branching approximation, $P_{\mathrm{fix}}\approx 2s$; inheritance, reproduction and competition assumptions matter.