Differences between HEB and LEB mice were estimated similarly to the inbred strain experiments as the percentage of total trait variation in head dipping accounted for by selected line. Correlated responses to selection were analyzed by ANOVA according to Crabbe et al. (37) where the strongest evidence for a genetic correlation is indicated by a significant difference in a trait between both replicates of the selected lines. Nonselected (“control”) lines of mice were not maintained, because these selected lines were designed for short-term use in the experiments reported here and were not intended as a long-enduring model for which nonselected control lines would be appropriate (37). Realized heritability was estimated according to Falconer and Mackay (38) as the ratio of the cumulative response to selection at each of the five generations of selection to the cumulative selection differential. The response to selection was expressed as a difference from the previous generation, and the cumulative selection differential represents the population-normalized deviation of the mice selected as breeders from the rest of the population.
