Down syndrome in two of three triplets.

This is the first report to our knowledge of trisomy 21 in two members of a set of triplets. The two with Down syndrome are monozygous; the other triplet is not identical. These triplets were born to a 22-year-old woman in whose family there have been six sets of twins (four like-sex) since 1775. Neither parent has any chromosomal abnormality. Zygosity estimates in the triplets are derived from studies of red blood cell antigens and HLA typing. Twinning has been reported in association with many different chromosome abnormalities. It has been suggested that the incidence of Down syndrome among twins is significantly higher than among singletons. It seems likely that some families have a heritable predisposition to both twinning and chromosome abnormalities.