An analysis of the response to selection for high and low rate of gain in the mouse, Mus musculus Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0z709107b

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  • An experiment was conducted to examine the responses to selection for high and low rate of gain in the mouse under a system whereby inbreeding was maximized. Rate of gain was measured by an index designed to include gains in both the pre- and postweaning periods with the gains in the latter period emphasized over those of the former. The various components (weights and gains) of the index were also analyzed in relation to their response to selection for the index. Data were collected on weight at 14, 24 and 32 days of age and litter size at 14 days of age. Results of the analyses indicated that while a response to selection for high index was obtained, virtually no response to selection for low index occurred. Responses of the component parts of the index (14, 24 and 32 day weights and gains between 14 and 24, 24 and 32 and 14 and 32 days) to selection for high index increased with increasing ages at which data were collected. Little if any response to selection for low index was obtained for the component parts of the index. It is proposed that the lack of response in the low line was caused by the opposition of natural selection to the artificial selection pressures imposed, which also was effective in maintaining a degree of heterozygosity above that described by calculated inbreeding coefficients. The hypothesis was supported by data on litter size. A disease which was widespread in the colony was found to have significantly reduced weights, gains and indexes in both the high and low lines. The disease was contributory in reducing the response to the two-way selection by effectively masking the phenotypic expression of genes for the several characteristics, A depressing effect of inbreeding on litter size occurred in both lines. The effect on the high line was more severe than that noted for the low line despite an increase in body size in the high line over five generations of selection. Thus inbreeding directly reduced litter size rather than indirectly through a reduction in maternal size, The cause(s) of the reduction were not determined. Random genetic drift due to small effective population sizes was hypothesized as the cause of the significant differences seen between replicates.
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