A 17-year-old Negro female with a 47, XX, 21 + karyotype gave birth to a premature male infant with clinical stigmas of Down's syndrome. His karyotype revealed a 47, XY, 21+ chromosomal composition. Mothers with trisomy-21 differ from those with trisoiny-X in that Down's syndrome mothers produce both normal and Down's syndrome progeny in approximately a 1:1 ratio; triplo-X mothers produce almost all normal children. The reason secondary nondisjunction is common in mothers with Down's syndrome but not in parents with sex chromosome triploidy is unclear. Prezygotic selection against the formation of a child with Down's syndrome does occur in the translocation D/G and 21/22 forms of the disease.
© 1970 The American College of Obstetricians and Gynecologists