The effects of inbreeding on the components of litter size in mice

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Abstract
Twenty-four lines were bred from a base population of outbred Q mice by continued full-sib mating. Inbreeding depression in litter size at birth was observed. This decline in litter size was analysed in terms of ovulation rate, the incidence of preimplantation mortality and the incidence of postimplantation mortality. Pregnant females were dissected at 17½ days' gestation and the numbers of corpora lutea, or eggs, and of live and dead embryos were counted. Matings were arranged so that separate estimates of the effects of inbreeding in the mother and in the litter on the components of litter size could be obtained.In the first generation of inbreeding when the inbreeding coefficient of the litter was raised from 0 to 25% decline in litter size was attributable to an increased incidence of preimplantation mortality.In the second and fourth generations decline in litter size was attributable to (1) a reduction in the number of eggs ovulated by the inbred mothers, (2) an increased incidence of preimplantation mortality which resulted from inbreeding in the mother. No evidence of significant effects on mortality of inbreeding in the litter was obtained in the later generation of inbreeding.These findings are discussed in the context of previous work on the effects of inbreeding and crossing on litter size and its components in mice and pigs.