In Table 4 are documented the first results concerning the parental and meiotic origin of trisomy 21 in the stud­ied German, Russian and Ukrainian families, together with the analysis of the epidemiological data. They have been prepared for the case-control study of trisomy 21 and other most frequent aneuploidies, to examine the impact of the Chernobyl accident, different lifestyles and environ­mental and genetic factors on meiosis I and II errors, and of their parental origin. The presented data demonstrate that, in the Russian and Ukrainian families, there is no different proportion of paternal non-disjunction estimated for 5-10% [39] relative to the proportion of maternally derived cases. The proportion of the meiosis I errors varies between 63% (Berlin) and 77% (Rostov, Kiev), thus around 70% in agreement with the studies of Petersen and Mikkelsen [40] and Hassold and Sherman [41], except for the families from Moscow (93.9%). Further analysis of more families, of the proportion of errors of meiosis I and II, of maternal and paternal origin together with the results of the case-control and epidemiological studies, are under way in order to find out the possible impact of the Cher­nobyl accident, and other factors that might endanger the meiosis process.