 Suppose that at a given locus, or chromosomal ‘slot’, there are two possible alleles, \(A_1\) and \(A_2\); the locus is assumed to be on an autosome, not a sex chromosome. With respect to the locus in question, there are three possible genotypes in the population, \(A_1 A_1, A_1 A_2\) and \(A_2 A_2\) (just as in Mendel’s pea plant example above). Organisms with the \(A_1 A_1\) and \(A_2 A_2\) genotypes are called homozygotes; those with the \(A_1 A_2\) genotype are heterozygotes. The proportions, or relative frequencies, of the three genotypes in the overall population may be denoted \(f(A_1 A_1), f(A_1 A_2)\) and \(f(A_2 A_2)\) respectively, where \(f(A_1 A_1) + f(A_1 A_2) + f(A_2 A_2) = 1\). It is assumed that these genotypic frequencies are the same for both males and females. The relative frequencies of the \(A\) and \(B\) alleles in the population are denoted \(p\) and \(q\), where \(p = f(A_1 A_1) + \frac{1}{2}f(A_1 A_2)\) and \(q = f(A_2 A_2) + \frac{1}{2}f(A_1 A_2).\) Note that \(p + q = 1\).