• Let us consider a single gene with two alleles, A and a, in a random mating population.
• There would be three genotypes, AA, Aa and aa, for this gene in the population. Suppose the population has N individuals of which D individuals are AA, H individuals are Aa and R individuals are aa so that D +H + R = N.
• The total number of alleles at this locus in the population would be 2N since each individual has two alleles at a single locus.
• The total number of A alleles would be 2D+H because AA individuals have two A alleles each, while each Aa individual has only one A allele.
• The ration (2D+H)/2N is, therefore, the frequency of A allele in the population, and is represented by p.
• Similarly, the ratio (2R + H) / 2N is the frequency of allele a, and is written as q.

Therefore, p = (2D + H) / 2N or = (D + ½ H) / N and

q = (2R + H) / 2N or = (R + ½ H) / N

Therefore, p + q = 1

and p = 1 – q,

or q = 1 – p

• The value of p and q are known as gene frequencies. Gene frequency is the proportion of an allele, A or a, in a random mating population.
• The genotype frequency or zygotic frequency is the proportion of a genotype, AA, Aa or aa, in the population.
• Random mating or random union of the two types of gametes would produce the following genotypes in a ratio proportionate to the frequencies of the gametes that united to produce them.