Allele frequency, or gene frequency, is the relative frequency of an allele (variant of a gene) at a particular locus in a population, expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in the population that carry that allele over the total population or sample size. Evolution is the change in allele frequencies that occurs over time within a population.

Given the following:

then the allele frequency is the fraction of all the occurrences i of that allele and the total number of chromosome copies across the population, i/(nN).

The allele frequency is distinct from the genotype frequency, although they are related, and allele frequencies can be calculated from genotype frequencies.

In population genetics, allele frequencies are used to describe the amount of variation at a particular locus or across multiple loci. When considering the ensemble of allele frequencies for many distinct loci, their distribution is called the allele frequency spectrum.

The actual frequency calculations depend on the ploidy of the species for autosomal genes.

The frequency (p) of an allele A is the fraction of the number of copies (i) of the A allele and the population or sample size (N), so

If ${\displaystyle f(\mathbf {AA} )}$, ${\displaystyle f(\mathbf {AB} )}$, and ${\displaystyle f(\mathbf {BB} )}$ are the frequencies of the three genotypes at a locus with two alleles, then the frequency p of the A-allele and the frequency q of the B-allele in the population are obtained by counting alleles.