The parental origin of the extra chromosome 21 and the meiotic failure involved were traced in 100 patients with Down syndrome and the relation with parental age was studied. In 20% of the cases the extra chromosome appeared to be of paternal origin (half of which caused by a nondisjunction at meiosis I, the remaining at meiosis II). In 80% the extra chromosome was of maternal origin (two thirds caused by a nondisjunction at meiosis I, one third at meiosis II). A significant increase of maternal age was only found in mothers who had the nondisjunction at meiosis II. In contrast, in fathers who experienced the nondisjunction at meiosis II the mean age appeared to be low. The possible causes of these phenomena are discussed.
Adult Age Factors