The effect of fast created inbreeding on litter size and body weights in mice.

This study was designed to reveal any differences in effects of fast created versus total inbreeding on reproduction and body weights in mice. A line selected for large litter size for 124 generations (H) and a control line (K) maintained without selection for the same number of generations were crossed (HK) and used as a basis for the experiment. Within the HK cross, full sib, cousin or random mating were practised for two generations in order to create new inbreeding (IB(F)) at a fast rate. In the first generation of systematic mating, old inbreeding was regenerated in addition to creation of new inbreeding from the mating design giving total inbreeding (IB(T)). The number of pups born alive (NBA) and body weights of the animals were then analysed by a model including both IB(T) and IB(F). The IB(T) of the dam was in the present study found to reduce the mean NBA with -0.48 (+/-0.22) (p < 0.05) pups per 10% increase in the inbreeding coefficient, while the additional effect of IB(F) was -0.42 (+/-0.27). For the trait NBA per female mated, the effect of IB(T) was estimated to be -0.45 (+/-0.29) per 10% increase in the inbreeding coefficient and the effect of IB(F) was -0.90 (+/-0.37) (p < 0.05) pups. In the present study, only small or non-significant effects of IB(F) of the dam could be found on sex-ratio and body weights at three and six weeks of age in a population already adjusted for IB(T).