Maternal and paternal origin of extra chromosome in trisomy 21

Fluorescence markers were studied in 40 patients with Down's syndrome and their parents. In 11 cases maternal and in 5 cases paternal non-disjunction could be shown. The disjunctional event occurred in the first meiotic division in 5 maternal and in 2 paternal cases. A second division failure was found in 4 maternal and 2 paternal cases. In 3 cases the failure could either be of first or second meiotic division origin.