Selection for increased 12-day litter weight in mice.

Selection for increased 12-day litter weight of standard litters of eight mice was practiced for nine generations in a replicated experiment with controls. The two selection lines followed very similar patterns of response. The pooled realized heritability was 0.08±0.04 and the observed genetic gain was 0.57 ± 0.19 g per generation. The replicated controls showed a negative trend in 12-day litter weight which was likely due to the effects of inbreeding and possibly some detrimental environmental fluctuation. Derivations of the components of variance affecting 12-day body weight indicated that direct additive genetic variance arising from genes controlling growth from birth to 12 days of age accounted for 37.0% of the total variation, while maternal additive genetic variance and postnatal maternal variance accounted for 16.4% and 49.6% of the variance, respectively. The direct additive genetic-maternal genetic covariance was negative but small. Significant positive correlated responses were observed for 12, 21, 42 and 56-day body weight, but no correlated responses were observed for postweaning body weight gain or for reproductive efficiency measured as number born and percent fertile matings.