Gene frequency measures the frequency in the population of a particular gene relative to other genes at its locus. Expressed as a proportion (between 0 and 1) or percentage (between 0 and 100%).
In the simplest case, gene frequency is measured by counting the frequencies of each gene in the population. If a genotype contains two genes, then there are a total of 16 genes per locus in a population of eight individuals:
Aa AA aa aa AA Aa AA Aa
In the population above,
frequency of A = 9/16 = 0.5625
frequency of a = 7/16 = 0.4375.
Algebraically, we can define p as the frequency of A and q as the frequency of a. p and q are always called 'gene' frequencies, but in a strict sense they are allele frequencies: they are the frequencies of the different alleles at one genetic locus.
The gene frequencies can be calculated from the genotype frequencies (P, Q, R ):
p = P + 1/2Q
q = R + 1/2Q
(and p + q = 1). The calculation of gene from genotype frequencies is highly important. Although the gene frequencies can be calculated from the genotype frequencies, the opposite is not true: the genotype frequencies cannot be calculated from the gene frequencies (p , q ).