Allele Frequency

Genetic diversity within a species depends on the variety of alleles in its gene pool. One way to describe this is the allele frequency.Allele frequency is the proportion of all the copies of a gene in a population that are a particular allele. It is written as a decimal between 0 and 1 (or as a percentage). The frequencies of all the alleles of a gene add up to 1.Because each diploid individual carries two copies of each gene, a population of N individuals contains 2N copies of the gene.allele frequency = number of copies of the allele / total number of copies of the gene (2N)Worked example. A population of 100 snails is sampled for a shell colour gene with alleles A and a. There are 36 AA, 48 Aa and 16 aa individuals.1. Total copies of the gene = 2 × 100 = 200.2. Copies of A = (2 × 36) + 48 = 120.3. Frequency of A = 120 / 200 = 0.6.4. Frequency of a = 1 − 0.6 = 0.4. (Check: (2 × 16) + 48 = 80, and 80 / 200 = 0.4.)Allele frequencies are useful because evolution is a change in allele frequency in a population over time. Natural selection, genetic drift, migration and mutation all change allele frequencies.A population in which one allele has a frequency of 1 has lost all other alleles of that gene, so it has low genetic diversity for that gene. A population where several alleles are present at similar frequencies has higher genetic diversity.