Genotype frequency refers to the proportion of a specific genotype within a population. It can be written in the form of an equation where the number of individuals with a given genotype are divided by the total number of individuals in the population.
In a population of size N, the genotype frequency = observed genotype / N.
Observed genotypes are either homozygote or heterozygote. In diploid organisms, there will be two homozygous possibilities and one heterozygous possibility. A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both chromosomes, and is heterozygous at a gene locus when its cells contain two different alleles of a gene. The number of each of the observed genotypes must add up to the total number of individuals in the population, and the percentages for each of the genotypes must add up to 100%.
Example: Observed genotypes: AA = 489, Aa = 537, and aa = 474
Population size, N = 1500
AA = 489 / 1500 = 0.326 = 32.6%
Aa = 537 / 1500 = 0.358 = 35.8%
aa = 474 / 1500 = 0.316 = 31.6%
32.6% + 35.8% + 31.6% = 100%