Assortative mating and the genetic correlation.

The effect of assortative mating on the genetic correlation between traits X and Y is considered. Assortation on trait X changes the magnitude of the genetic correlation but not its sign. There are two situations depending on the signs of the correlation between mates (ρ) and of the random mating genetic correlation (θ): 1) if sign (θ) = sign (ρ), then θ >θ, where θ is the genetic correlation at equilibrium after continued assortation, and 2) if sign (θ) ≠ = sign (ρ), then θ < θ. However, negative assortative mating is virtually powerless to alter the magnitude of the genetic correlation. The consequences of a "mixed" assortation model, e.g., high milk production females mated to fast growing males and lesser productive females mated to slower growing sires, were also studied. "Mixed" positive assortation always increases the genetic correlation, but negative assortation decreases it. The implications of assortative mating on correlated responses to selection and on the equilibrium covariances between relatives for pairs of traits are discussed.