Editing Allele frequency (section)

{{short description|Relative frequency of a variant of a gene at a particular locus in a population}}
'''Allele frequency''', or '''gene frequency''', is the relative frequency of an [[allele]] (variant of a [[gene]]) at a particular [[locus (genetics)|locus]] in a [[population]], expressed as a fraction or percentage.<ref name="gillespie">{{cite book|last1=Gillespie|first1=John H.|title=Population genetics : a concise guide|date=2004|publisher=The Johns Hopkins University Press|location=Baltimore, Md.|isbn=978-0801880087|edition=2.}}</ref> 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 cell]]s (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.<ref name="gillespie" />

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]].