Allele frequency refers to the different forms of a gene at a single position on a chromosome. It is the proportion of a given allele to all alleles at the same locus. It is usually expressed as a percentage.

Allele frequencies may be used to describe genetic diversity in a population. If the frequency of any allele is above 1% it is probable that natural selection is maintaining it at this level. This is because the natural mutation rate of an allele is almost always much lower than 1%.