In a population of animals we can calculate the allele frequencies and the genotype frequencies for a monogenic trait. This is of value when you have an animal with known alleles for a certain monogenic trait and you want to calculate the chance for finding another animal (for mating) with a desired genotype for this trait.
Suppose a monogenic trait has two alleles Z and z. Then animals will have one out of the three genotypes possible: Z/Z, Z/z or z/z. Example: in a population of 630 animals we count 375 animals with the genotype Z/Z, 218 with the genotype Z/z and 37 with the genotype z/z. The frequency of the three genotypes in the population is: 375/630 = 0.595; 218/630 = 0.346 en 37/630 = 0.059.
The allele frequencies can be calculated as follows: Z/Z animals do have 2 Z alleles; Z/z animals do have 1 Z allele and z/z do have 0 Z alleles. Thus the frequency of the Z allele is: 0.595 + 0.5 * 0.346 = 0.768. The Z/z animals do have 1 z allele and the z/z animals do have two z alleles. Thus the frequency of the z allele is: 0,5 * 0,346 + 0,059 = 0,232. In population genetics the frequency of alleles is noted as the frequency of p and the frequency of q. In this case p = 0.768 and q is equal to 0.232. The sum of p and q is always equal to 1 (in this example: 0,768 + 0,232).