Allele Frequency

Allele frequency is the proportion of all copies of a gene in a population that are a particular allele. It is usually written as a number between 0 and 1, and the change in allele frequency over generations is the technical definition of evolution.

Count the copies of the allele and divide by the total number of copies of the gene. In a diploid population of 100 people, there are 200 copies. If 40 of them are allele A, its frequency is 0.2. If a gene has two alleles, their frequencies p and q add up to one, which is the starting point for the Hardy-Weinberg principle.

Four forces change allele frequencies. Natural selection favors alleles that raise fitness, and mutation introduces new alleles. Gene flow brings alleles between populations through migration, and genetic drift causes random changes, especially in small groups, as in the founder effect. The size of the breeding group is described by the effective population size.

The allele for sickle hemoglobin reaches frequencies of 10 percent or more in parts of Africa where malaria is common, but is rare elsewhere. Allele frequencies are measured by DNA sequencing and compared between populations in population genetics, and large databases now list them for millions of human variants.

• Allele — what is being counted
• Hardy-Weinberg Principle — the null model for frequencies
• Genetic Drift — a chance cause of change