 An important result in two-locus theory shows that, given random mating, the quantity \((x_2 x_3 - x_1 x_4)\) will decrease every generation until it reaches zero—at which point the genotype frequencies will be in equilibrium. So a population initially in linkage disequilibrium will approach linkage equilibrium over a number of generations, at a rate that depends on \(r\), the recombination fraction. Note the contrast with the one-locus case, where just one round of random mating is sufficient to bring the genotype frequencies into equilibrium.