Statistical analysis
The MEANS and UNIVARIATE procedures of SAS were used to generate descriptive statistics and test normality assumptions for each trait [55]. All data were then analyzed using the GLM procedure of SAS [55] with a linear model that included the fixed effects of strain, sex and strain by sex interaction; dam's weight at breeding by strain was used as a covariate. A second linear model was used to test for strain by hg genotype (+/+ or hg/hg) interactions. This model included the fixed effects of donor region, sex and HG genotype and all possible two and three-way interactions. Choosing a nominal P value of 0.05 and applying the Bonferroni correction for multiple comparisons established significant differences in the ANOVA's. The critical P values used are indicated in each table.