Study of parental/meiotic origin of free trisomy 21 in nuclear families from Russia (70 cases), Ukraine (32 cases), and 22 from Germany revealed maternal nondisjunction in 77.3% (Germany), 93.8% (Ukraine), and 91.4% (Russia), paternal origin in 13.6%, 6.2%, and 8.6%, respectively. Maternal meiosis I errors were found in 84.4% (Ukraine), 77.1% (Russia), paternal origin in 3.1% (Ukraine), 2.9% (Russia). Maternal meiosis II errors occurred in 9.4% and 14.3% and paternal in 3.1% and 5.7% in Ukraine and Russia, respectively. No significant differences were found in maternal/paternal origin among Ukraine, Russia, Germany, and published data from other European regions.

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