 From the recurrence equations, it follows that gametic (and thus genotypic) frequencies will be stable across generations, i.e., \(x_i' = x_i\) for each \(i\), under either of two conditions: (i) \(r = 0\), or (ii) \(x_2 x_3 - x_1 x_4 = 0\). Condition (i) means that the two loci are perfectly linked and thus in effect behave as one; condition (ii) means that the two loci are in ‘linkage equilibrium’, so that alleles at the \(A\)-locus are in random association with alleles at the \(B\)-locus. More precisely, linkage equilibrium means that the population-wide frequency of the \(A_i B_i\) gamete is equal to the frequency of the \(A_i\) allele multiplied by the frequency of the \(B_i\) allele.