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

North Carolina State University⁴, Raleigh 27695-7621

Abstract

Genetic differences in performance of dams were estimated by linear contrasts using means of two selected lines of mice and reciprocal F₁ 's, F₂ 's and backcrosses. The lines were selected for increased 6-wk body weight (W) or increased litter size (L). Genetic differences estimated were direct average (g_D), direct heterosis (h_D), maternal average (g_M), progeny average (g_P), and progeny heterosis (h_P). For dam weight and feed consumption from 12 to 21 d postpartum (pp), g_D was the largest genetic difference and favored line W. For litter size, litter weight at birth, litter efficiency (litter weight gain/dam feed consumption) from birth to 12 d pp and within litter mortality from 1 to 21 d pp, g_D favored L and, except for h_D in litter efficiency, was the most important genetic difference for these traits. Direct heterosis was the only significant difference for litter weight at 21 d pp, litter efficiency from 12 to 21 d pp and within litter mortality at parturition. The g_M were larger in W than in L for dam weight and feed consumption, and for litter size and weight at birth, but they were usually of smaller magnitude than g^D. The g_P were significant only in litter traits measured before 12 d pp and favored W. For no trait measured was h_P of consequence. Line differences in dam and litter weight accounted for genetic differences in dam feed consumption. Genetic differences in litter size at birth were not due to line differences in dam weight. The lower mortality within litters nursed by crossbred dams was responsible for h_D on litter weight and litter efficiency. Within but not among lines, higher mortality rates were associated with larger litters.

¹ Paper No. 8932 of the Journal Series of the North Carblina 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 and Mr. P Fine. Suggestions from Dr. M. W. Tess were helpful in the preparation of this manuscript.

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

⁴ Dept. of Anim. Sci.