The allele frequency is the proportion of all the alleles of a gene in a population that are a particular allele. It is a number between 0 and 1, and the frequencies of all the alleles of a gene add up to 1.Allele frequency can be calculated directly when every genotype can be identified, for example with codominant alleles, or when genotypes are known from DNA testing. Each individual carries two alleles, so the total number of alleles is twice the number of individuals.allele frequency = number of copies of the allele / total number of alleles of that gene Worked example: a population of 100 snails contains 36 AA, 48 Aa and 16 aa.Total alleles = 100 × 2 = 200
Copies of A = (36 × 2) + 48 = 120
Copies of a = (16 × 2) + 48 = 80Frequency of A = 120 / 200 = 0.6
Frequency of a = 80 / 200 = 0.4
Check: 0.6 + 0.4 = 1Note that allele frequency is not the same as genotype frequency or phenotype frequency. In this population the genotype frequency of aa is 16 / 100 = 0.16, but the frequency of the a allele is 0.4, because many a alleles are hidden in heterozygotes.When a dominant allele hides the recessive one, heterozygotes cannot be told apart from homozygous dominants, so allele frequencies cannot be counted directly. They must then be estimated from the frequency of the recessive phenotype using the Hardy-Weinberg equation.
Copies of A = (36 × 2) + 48 = 120
Copies of a = (16 × 2) + 48 = 80Frequency of A = 120 / 200 = 0.6
Frequency of a = 80 / 200 = 0.4
Check: 0.6 + 0.4 = 1Note that allele frequency is not the same as genotype frequency or phenotype frequency. In this population the genotype frequency of aa is 16 / 100 = 0.16, but the frequency of the a allele is 0.4, because many a alleles are hidden in heterozygotes.When a dominant allele hides the recessive one, heterozygotes cannot be told apart from homozygous dominants, so allele frequencies cannot be counted directly. They must then be estimated from the frequency of the recessive phenotype using the Hardy-Weinberg equation.