The capacity of heterozygous yellow-lethal (Aya) and brown (aa) male mice to induce the Bruce effect was investigated. The proportion of SJL/J females showing a failure of implantation when exposed to males of either genotype was 80 %, whereas only 20 % of the control females not so exposed showed pregnancy failure. One-half of the females whose pregnancy had been blocked as a result of exposure to either the Aya or aa male mice (50 % and 55 %, respectively) ovulated within 7 to 8 days after the initial mating. Results are discussed in regard to the hypothesis that the neuroendocrine mechanisms involved in pregnancy blocking and synchrony of oestrus are not qualitatively different.
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