Cytogenetic analysis of the first cleavage division in mouse embryos (fertilization-pronuclei-T163H translocation).

Chromosome counts of 338 mouse zygotes at late prophase and metaphase of the first cleavage division revealed 96.4% diploidy, 1.8% hypodiploidy, 1.2% triploidy, and 0.3% tetraploidy. One additional anomaly might have given rise to an embryo mosaic for maternal but not paternal genes. In zygotes collected without colchicine administration to the mother, one of ten anaphases had a lagging chromosome. Reciprocal crosses with the translation T163H as a marker chromosome at late prophase, when the male- and female-derived chromosome groups have not yet combined, demonstrate that the group derived from the larger pronucleus and having less condensed chromosomes is of paternal origin. T163H undergoes Mendelian segregation in female heterozygotes.