Flexi Says: Allele frequency is how often an allele occurs in a gene pool relative to the other alleles for that gene. In a sexually reproducing species, each member of the population has two copies of each gene. For example, in a population of 100, the total number of copies of each gene in the gene pool is 200. If you know the genotypes of each population member, you can count the number of alleles of each type in the gene pool. Remember that homozygous individuals contribute two of the same allele, while heterozygous individuals contribute one of each allele. Let the letter p stand for the frequency of the dominant allele (e.g., A), and the letter q stand for the frequency of the recessive allele allele (a). In a sample population of 100 individuals with 140 A alleles and 60 a alleles, you can calculate p and q as follows:
p = number of A alleles/total number of alleles = 140/200 = 0.7
q = number of a alleles/total number of alleles = 60/200 = 0.3
Notice that p + q = 1.