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Direct and Maternal Genetic Differences between Lines of Mice Selected for Body Weight and Litter Size: Traits of Offspring^1,2,

North Carolina State University⁴, Raleigh 27695-7621

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 (g_D), direct heterosis (h_D), maternal average (g_M), maternal heterosis (h_M) and recombination loss (r_D). Litters were not standardized at birth. For body weights and gains, g_D or g_M was the most important genetic difference at all ages, and W was the line favored. At birth, 12 d and 21 d, g_M was the only significant genetic difference, but g_D was more important than g_M at 42 d. The only significant genetic difference for gain from 21 to 42 d was g_D. 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 g_D and g_M 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 g_D, 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 g_D, g_M and h_M, the former two favoring W and the latter being negative. Efficiency of postweaning gain showed differences in g_D and g_M. Line W was favored only for the former. Efficiency of protein gain from 21 to 42 d did not show any genetic differences.

¹ Paper No. 8966 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned.

² The authors are thankful for the technical assistance of Ms. B. J. Edwards, Mr. R. Bruner and Mr. P. Fine and the use of the research facilities of Drs. J. M. Leatherwood and W. F. McClure and Mr. D. Herman.

³ Present address: Dept. of Anim. Sci., Purdue Univ., Lafayette, IN 47907.

⁴ Dept. of Anim. Sci.