 The one-locus model outlined above is unrealistic, since in practice evolution may occur at multiple loci simultaneously. The simplest two-locus model assumes two autosomal loci, \(A\) and \(B\), each with two alleles, \(A_1\) and \(A_2, B_1\) and \(B_2\). Thus there are four types of haploid gamete in the population—\(A_1 B_1, A_1 B_2, A_2 B_1\) and \(A_2 B_2\)—whose frequencies we will denote by \(x_1, x_2, x_3\) and \(x_4\) respectively. (Note that the \(x_i\) are not allele frequencies; in the two-locus case, we cannot equate ‘gamete frequency’ with ‘allelic frequency’, as is possible for a single locus.) Diploid organisms are formed by the fusion of two gametes, as before. Thus there are ten possible diploid genotypes in the population—found by taking every gamete type in combination with every other.