Allele frequency

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.^[1] Specifically, it is the fraction of all chromosomes in the population that carry that allele over the total population or sample size. Microevolution is the change in allele frequencies that occurs over time within a population.

Given the following:

A particular locus on a chromosome and a given allele at that locus
A population of N individuals with ploidy n, i.e. an individual carries n copies of each chromosome in their somatic cells (e.g. two chromosomes in the cells of diploid species)
The allele exists in i chromosomes in the population

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.^[1]

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.

^ ^a ^b Gillespie, John H. (2004). Population genetics : a concise guide (2. ed.). Baltimore, Md.: The Johns Hopkins University Press. ISBN 978-0801880087.

[gillespie-1] Gillespie, John H. (2004). Population genetics : a concise guide (2. ed.). Baltimore, Md.: The Johns Hopkins University Press. ISBN 978-0801880087.

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