Parental origin of the extra chromosome 18 in Edwards syndrome.

Trisomy 18 (Edwards syndrome) is the second most common trisomy in human. The parental origin of the additional chromosome leading to meiotic errors is imperative to understand the etiology of trisomy 18. We compiled the data on 171 cases (table I) where meiotic error is provided. The paternal meiotic error occurred in 7% of the cases while in 93% of the cases nondisjunction took place in the mother. There were 28 cases (16%) where nondisjunction took place in meiosis I(MI), 53 cases (31%) in meiosis II (MII) and in 90 cases (53%) either the error was postzygotic mitosis or the dysfunctional stages could not be determined. The age distribution could be found in 119 cases (table II) and a conclusion could not be drawn as there were only 11 cases where paternal nondisjunction was noted.