Meiosis I non-disjunction as the main cause of trisomy 21.

The relative roles of Meiosis I and Meiosis II non-disjunctions in the causation of trisomy 21 have been assessed by analysing the distribution of polymorphic phenotypes of the chromosomes 21 in a group of individuals with Down's syndrome. The data suggest that the majority of cases of trisomy 21 are due to meiosis I non-disjunctions.