Published online by Cambridge University Press: 02 September 2010
An unselected, non-inbred population of laboratory mice has been propagated for 25 generations by 20 pairs (families) of mice, each leaving one son and one daughter selected at random from the litter. The daughter was given the family number of her dam, while her mate came from another family varying systematically in different generations. The breeding plan precluded any possibility of common ancestry for five generations. In later generations the inbreeding coefficient rose at an average rate expected from a population with an effective number of approximately 80, to approximately 0·13 in generation 25. The rise occurred in cycles, repeating every five generations. Data are presented to demonstrate that overall reproduction (total weight of young produced at 9 weeks of age, per female mated) and the components of survival from birth to 9 weeks, survival from sexing (3 weeks) to 9 weeks separately for males and females, and 9-week weight of males and females, were each sensitive to changes in inbreeding coefficient. In contrast, conception rate, litter size (live young born) and sex ratio did not reflect changes in the inbreeding coefficient. The magnitude of the depression in overall reproduction was substantially greater than that in any of the affected components.