A cytogenetic study of G1-chromosomes in one-cell stage mouse embryo and in corresponding second polar body. Evaluation of aneuploidy originated in females heterozygous for translocation T[14;15]6Ca.

Meiotic chromosomes in ovulated oocytes and G1-chromosome sets visualized in the 2nd PB and in the pronuclei of one-cell stage embryos treated with okadaic acid were studied in female mouse heterozygous for reciprocal translocation T[14;15]6Ca. It was found that 61.5% of oocytes were haploid, 14.9% hyperhaploid and 23.6% hypohaploid. Unpaired chromatid (a half-dyad), in addition to (or replacing) a whole chromosome (a dyad), was detected in 20% of oocytes. G1-chromosome complements in the 2nd PB and in the MPN expected in the case of aneuploidy due to chromosome non-disjunction or to chromatid abnormal segregation at the first and second meiotic division were detected in one-cell stage embryos. The hypo- and hyperhaploidy caused by non-disjunction was revealed in 17.6% of embryos. Aneuploidy due to abnormal segregation of a chromatid in the first and the second meiotic division was in 20% and 4.4% of all cases respectively. The incidence of different classes of aneuploid oocytes were almost fully conformable to that of corresponding types of aneuploidy detected in one-cell stage embryos. The main advantage of the proposed new approach based on cytogenetic analysis of G1-chromosomes in the 2nd PB and in the corresponding MPN is that it allows to study not only the chromosome non-disjunction, but also abnormal segregation of chromatids in the first and in the second meiotic divisions, and to estimate accurately the incidence of these maternal meiotic errors.