Allele frequency

Allele frequency is the proportion of the copies of a gene in a population that are one particular allele of that gene.

This can be expressed as a fraction, decimal or percentage. For a given allele, X, found at a locus, Y, the allele frequency of X would mathematically be represented as:

(number of copies of allele X in the population)/(total number of copies of all the alleles found at locus Y in the population).

So, for instance, if in a population of 300 diploid organisms, a single genetic locus has three alleles - a, b and c - and allele a appears 174 times then, given that there must be 600 copies of all the alleles for this locus in total (300 x 2 loci, due to diploidy), the allele frequency of allele a is 174/600 or 29%.

Changes in allele frequency are the basis of evolution. Such changes are initially caused by random mutation, which may either eliminate existing alleles or introduce new ones, and the effect of those mutations may be shaped by any combination of natural selection, genetic drift or gene flow.