 Secondly, positive assortative mating will tend to decrease the proportion of heterozygotes in the population,. To see this, consider again a single locus with two alleles, \(A_1\) and \(A_2\), with frequencies \(p\) and \(q\) in a given population. Suppose that initially the population is at Hardy-Weinberg equilibrium, so the proportion of \(A_1 A_2\) heterozygotes is \(2pq\). If the population then starts to mate completely assortatively, i.e., mating only occurs between organisms of identical genotype, it is obvious that the proportion of heterozygotes must decline. For \(A_1 A_1 \times A_1 A_1\) and \(A_2 A_2 \times A_2 A_2\) matings will produce no heterozygotes; and only half the progeny of \(A_1 A_2 \times A_1 A_2\) matings will be heterozygotic (on average). So the proportion of heterozygotes in the second generation must be less than \(2pq\). Conversely, negative assortment will tend to increase the proportion of heterozygotes from what it would be under Hardy-Weinberg equilibrium.