Abstract
Genetic differences in growth and body composition were estimated from linear contrasts among two selected lines of mice, reciprocal F1′s, F2′s and backcrosses. Line W had been selected for increased 6-wk body weight and line L for large litter size. Genetic differences estimated were direct average (gD), direct heterosis (hD), maternal average (gM), maternal heterosis (hM) and recombination loss (rD). Litters were not standardized at birth. For body weights and gains, gD or gM was the most important genetic difference at all ages, and W was the line favored. At birth, 12 d and 21 d, gM was the only significant genetic difference, but gD was more important than gM at 42 d. The only significant genetic difference for gain from 21 to 42 d was gD. Average maternal differences were due to the more favorable environment resulting from the smaller litter size of W dams. Direct heterosis was not significant at any age. However, differences in litter size had suppressed a direct heterotic advantage for weight at 21 and 42 d and for postweaning gain. At 21 d, the larger litter size of crossbred dams prevented expression of their better maternal influence on progeny weight. Maternal heterosis for 42-d weight was actually negative as a result of the negative environmental impact of increased litter size. These selection criteria have resulted in up to 9 and 10% direct and maternal heterosis, respectively, on body weight, but standardization of litter size would be required for it to be expressed. Recombination loss was not significant for weight at any age. For weight of ash, fat, moisture and protein at 42 d, differences in gD and gM were similar and favored line W. Direct heterosis was observed for all constituents except fat. When expressed as a percentage of the whole body, only fat and protein exhibited a genetic difference. For gD, line W had a higher percentage fat, but L had a higher percentage protein. Significant differences in 21 to 42 d feed consumption were found for gD, gM and hM, the former two favoring W and the latter being negative. Efficiency of postweaning gain showed differences in gD and gM. Line W was favored only for the former. Efficiency of protein gain from 21 to 42 d did not show any genetic differences. Copyright © 1984. American Society of Animal Science . Copyright 1984 by American Society of Animal Science