Average genetic and heterotic effects on growth in mice selected for large 6-week body weight or rapid postweaning gain.

Data on body weights at 3 weeks (WK3) and 6 weeks (WK6) and postweaning gain from 3 to 6 weeks (GAIN) were analyzed in eight populations of mice to estimate the differences between populat ions in average direct genetic effects (ADG), average maternal genetic effects (AMG) and direct heterotic effects (DH). The effects of population, sex, group size and interactions were examined. Females were caged singly or in groups of four. Populations studied were M16, selected for GAIN, H6, selected for WK6, their respective base population controls (ICR and C2), reciprocal Fi crosses betweeh M16 and H6, and reciprocal Fi crosses between ICR and C2. The positive (P<.O1) responses to selection in H 6 relative to the C2 control were 21.7, 33.7 and 46.4% for WK3, WK6 and GAIN, respectively. Corresponding positive (P<.O1) responses for these traits in M16 were 28.0, 49.5 and 70.1% greater than the ICR control. ADG accounted for the major part of this greater selection response in M16 compared to H 6. AMG were not responsible for any significant selection response difference between M16 and H6. Differences between the controls (ICR minus C2), expressed as a percentage of the parental means, were 15.4% (P<.01) for WK3 (2.8% for ADG, P>.05 and 12.7% for AMG, P<.01), 20.3% (P<.01)