Allele frequency (proportion of gene variants in populations)

How it is calculated

For diploid organisms each individual carries two copies of a locus, so the total number of gene copies equals twice the number of individuals sampled. The frequency of allele A is calculated as the number of A copies divided by the total number of copies at that locus. For example, in 10 diploid individuals there are 20 total copies; if allele A appears 12 times its frequency is 12/20 = 0.6 (60%). Simple counting like this underlies more complex estimates from genotype data.

Processes that change allele frequency

Allele frequencies shift through several evolutionary forces. New variants arise by mutation, but typical per‑locus mutation rates are low, so mutation alone usually changes frequencies slowly. Natural selection alters frequencies when alleles affect survival or reproduction. Random sampling effects (genetic drift) can move frequencies, especially in small populations. Migration or gene flow brings alleles in or out, and nonrandom mating can redistribute genotype frequencies without changing allele proportions initially.

Equilibrium, fixation and loss

Under the assumptions of Hardy–Weinberg equilibrium (no selection, mutation, migration, drift or nonrandom mating), allele frequencies remain constant across generations and genotype proportions can be predicted (for two alleles with frequencies p and q: p², 2pq and q²). An allele reaches fixation when its frequency becomes 1.0 (every chromosome carries it), and it is lost when frequency reaches 0.0. Both fixation and loss can result from selection or drift.

Uses, measurement and caveats

Measuring allele frequencies is central to studies of genetic diversity, conservation genetics, association studies and forensic genetics. Frequencies are estimated from samples using genotyping arrays or DNA sequencing; sampling error, population structure, and small sample sizes can bias estimates. When a previously rare allele is observed at appreciable frequency (for example, above about 1%), it is often an indication that forces other than new mutation—such as selection or migration—have influenced its rise, because new mutation alone is usually insufficient to maintain high abundance.

Practical examples and notable facts

• Human blood group alleles vary among regions; allele frequencies help predict genotype and phenotype distributions.
• Conservationists track allele frequencies to assess loss of genetic diversity in endangered species.
• Population bottlenecks can shift frequencies rapidly through drift, sometimes fixing deleterious alleles.

Understanding allele frequency provides a quantitative foundation for evolutionary and applied genetics. For further reading on related terms see gene, chromosome, allele, locus, percentage, genetic, population, natural selection and mutation.