Gene frequency is a measure of how common a particular allele is as a proportion of all possible alleles at a given locus (a physical location on a chromosome) within a population. This measure is also referred to as allelic frequency.

If a given allele exists in a population in two different forms "A" and "a", then the percentage of "A" taken as a measure of the overall occurrence of both "A" and "a" would be the gene frequency for "A" in that population. So, for example, if there are 30 individuals with a genotype "AA," 50 with "Aa," and 20 with "aa," then it can be seen that "A" is present a total of 110 times (30 + 30 + 50). This is from a total possible occurrence of 200 (30 + 30 + 50 + 50 + 20 + 20) alleles at the locus under study. The gene frequency in this case for "A" is 110 / 200 = 0.55 or 55%.

Gene frequency can only be quoted for a population; the population can, however, be of any number of individuals. Gene frequency can, in certain circumstances, only be an estimate due to the hidden, recessive alleles that occur in populations.