ABSTRACT
TRISOM Y 21 Down's syndrome is the commonest viable autosomal trisomy (Figure 1). The incidence increases from 1 per 1000 live births in mothers under 30 years, to 1 in 30 live births in those aged 45 years. Mothers who have had one affected child are at increased risk of having a sec ond, implying a predisposition to nondisjunction in meiosis I. Approximately 30% of the mentally handi capped people in Britain suffer from Downs syndrome.
