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The factors in the eggs of DDK mice and in alien spermatozoa which are responsible for embryonic death in DDK females mated with males from other strains were subjected to genetic analysis. (1) In reciprocal backcrosses involving DDK mice and three other inbred strains, embryonic survival was halved when DDK females were mated with F1 males, or when F1 females of DDK and two other strains were mated with males of the corresponding strains, compared to the results of matings of F1 females with DDK males. (2) Reduction of litter size in the semi-sterile backcrosses was found to be caused by embryonic death at the morula-blastocyst and pre-egg cylinder stages at or around the time of implantation. A common feature of the dead embryos was a defect in trophoblast formation. (3) Genetic analysis suggests that the cytoplasmic factor of the egg and the factor in spermatozoa are both controlled by autosomal genes located at the same locus or closely linked.The gene symbol om (ovum mutant) has been given to the mutant gene possessed by the DDK strain, and it is proposed that the cytoplasmic factor of the egg interacts specifically with the gene of sperm origin to synthesize a substance which is necessary for trophoblast formation.

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