Consider a single gene with two alleles, as in Mendel’s monohybrid cross. An individual may have the genotype AA, Aa or aa. In a population, the proportion of A alleles is defined as p, while the frequency of a alleles in the population is defined as q. The A alleles and the a alleles together make up all the alleles of gene A in the population or, p + q = 1. Since the proportion of AA individuals in a population would be equal to the probability of getting an A allele from one parent and a second A allele from another, and since the proportion of A alleles is equal to p, the proportion of AA individuals is equal to p2. Reasoning in the same manner, the proportion of aa individuals in a population is equal to q2 and the proportion of heterozygotes, Aa, is equal to 2pq.
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