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Nondisjunction in the first meiotic division was evaluated by the frequency of aneuploid metaphase II (M II) figures in secondary spermatocytes of Mus musculus (N = 40; FN = 40), M. poschiaυinus (N = 26; FN = 40) and M. musculus × M. poschiavinus F₁-hybrids (N = 33; FN = 40), whose karyotypes are related by Robertsonian-type centric fusions. Regular anaphase I disjunction prevailed in M. musculus and M. poschiavinus, where 92.8% and 94.0% of the M II figures examined had a complete haploid chromosome complement. In the F₁-hybrids, however, less than one half of the M II figures examined were euploid (FN/2 = 20), whereas the frequency of hypomodal (FN/2 = 15 to 19; 31.2%) and hypermodal (FN/2 = 21 to 25; 24.2%) was high. These results emphasize the importance of the heterozygous state, and particularly structural heterozygosity within a Robertsonian system of karyotypic variation, in inducing meiotic nondisjunction. From the standpoint of methodology, M. musculus × M. poschiavinus hybrids of different generations should make possible stepwise analysis of the involvement of specific chromosomes in meiotic nondisjunction.

Nine new cases of prenatally detected true mosaic trisomy 20 (T20) are reported. In three instances the fetuses were aborted. One fetus showed multiple malformations associated with a high percentage of T20 cells among amniotic fluid (AF) cells and fibroblasts of different fetal tissues. In two other fetuses only a slight facial dysmorphy was seen which was accompanied by a low percentage of T20 cells among AF cells. In five instances the pregnancies were carried to term, and normal somatic and psychomotor development of the children has been observed, in one case up to the age of 24 months. In one case the pregnancy is continuing. The T20 cells were not detected among cultured lymphocytes of these children. A review of the hitherto known cases of prenatally detected mosaic T20 indicates a relationship between the prenatal findings and the fetal development. This may serve as a provisory basis for genetic counselling: in the case of a percentage above 50% of T20 cells among AF cells there seems to be a risk of about 50% for the fetus to be affected by severe anomalies. However, in cases of a prenatally detected mosaic T20 with a percentage equal to or less than 50, fetal or congenital malformations have not been observed among 23 individuals so far examined.

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A sharp reduction in fertility of the F1 hybrids between the tobacco mouse (M. poschiavinus) and the laboratory mouse can be attributed to irregular segregation of trivalents in meiotic anaphase I. In male meiosis, more than 50% of aneuploid secondary spermatocytes are produced. The cytophotometric measurements of the relative Feulgen-DNA content of cytologically mature, morphologically normal spermatozoa revealed a significantly greater variation in F1 males as compared to M. poschiavinus and the laboratory mouse. This difference is regarded as being due to the presence of a considerable fraction of aneuploid spermatozoa. Obviously, they are derived from the aneuploid precursor cells and have developed normally in spite of their abnormal number of chromosomes. However, the frequency of malformed spermatozoa was also higher in F1 males than in the parental strains. The additional analysis of the number of spermatozoa, their motility and the respective proportions of living or dead spermatozoa and of spermatozoa with normal or abnormal acrosomal caps did not show differences between F1 males and the parental strains. The head area of F1 spermatozoa was markedly reduced.