The mean and variance of first parity litters of 8 control populations of mice of different origins, each maintained in two environments were examined (N=22422). The main populations were maintained with 25 single pair matings for 70 generations in a controlled environment. A replicate was sampled from each population around generation 18 and maintained with 15 single pair matings in an uncontrolled environment. The populations were crosses between two and four inbred lines, non-inbred strains and their crosses. The mean and variance of litter size in the replicates were significantly larger in almost half of the cases than those in the main populations. Despite significant differences in litter size between generations in almost all the populations, the between generations component of variance was much smaller than the within generations component of variance in every case. A fairly good agreement existed between the expected genetic heterogeneity of the populations arid their within generation mean-squares. 

A. Farid, M. Makarechian, J. A Newman

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XII. Biotechnology, selection experiments, parameter estimation, design of breeding systems, management of genetic resources., , 333–338, 1986
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