Meiosis in trisomic female mice with Robertsonian translocations. II. Chromosome behaviour at first and second meiotic metaphases.

First and second meiotic metaphases (MI and MII, respectively) from female mice of Robertsonian translocation (Rb) stock, trisomic for chromosome 16 (Ts16) or 19 (Ts19), were studied. The mature trisomic oocytes were derived from explanted fetal ovaries that had been cultured and then transplanted so as to mature heterotopically. Multivalent configurations involving the Rb chromosomes and the additional trisomic acrocentric were analysed. Pentavalent configurations occurred in 74.5% of 98 Ts16 MI and 44.2% of 249 Ts19 MI oocytes; quadrivalents (with a univalent acrocentric) were found in 9.2% of Ts16 MI and 10.8% of Ts19 MI oocytes. In 1% of Ts16 MI and 4% of Ts19 MI oocytes, there were two Rb bivalents and a univalent trisomic acrocentric. Rb trivalents and Rb bivalents occurred together in 14.3% of Ts16 MI and 39.4% of Ts19 MI oocytes. Chiasma frequencies were similar in trisomic and chromosomally balanced MI. Chiasma position, distribution, and localization were nearly identical, whether they were found in Rb multivalents or acrocentric bivalents, but one control group (from chromosomally balanced Ts19 littermates) had significantly more terminal chiasmata. Within the triple homologous region of 8% of Rb pentavalents, two chiasmata were observed in the same relative position in the two sister chromatids of one of the three homologs, suggesting a lapse in chiasma position interference. Assortment at MI anaphase was influenced by secondary nondisjunction of the Rb. The ratio of balanced to unbalanced MII oocytes was 1:4 in both trisomies.