DIRECT AND MATERNAL GENETIC RESPONSES RESULTING FROM SELECTION FOR 12-DAY LITTER WEIGHT IN MICE

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Abstract
Response to selection for 12-day litter weight in mice was evaluated in terms of average direct and average maternal genetic responses. Comparisons were made among selected and control parental lines, reciprocal F1crosses, F2's and backcrosses. Selection response in 12-day body weight was primarily due to average direct genetic effects. Correlated responses in body weight at 21, 31, 42 and 70 days also were due to average direct effects. Average maternal genetic effects contributed little to direct or correlated weight responses. Heterosis for direct genetic effects was not significant for 12- and 21-day body weights but was important for postweaning weights. In contrast, heterosis for maternal genetic effects was important at 12 and 21 days but declined after weaning. These results suggest that the maternal genetic influence on neonatal mammalian growth may play a key role in fitness of the offspring. Thus, natural selection would tend to reduce additive maternal genetic variance, leaving mainly non-additive maternal genetic variance. This, coupled with the large maternal environmental component influencing neonatal growth, suggests that selection for additive direct effects influencing preweaning growth in livestock would be most efficiently carried out by standardizing or eliminating the postnatal maternal environment.