Recurrent regular trisomy-21 in two Bedouin families. Parental mosaicism versus genetic predisposition.

Two young, unrelated multiplex families with Bedouin ancestors, each confirmed to have 3 sibs with recurrent regular trisomy-21 are reported. Low grade mosaicism for trisomy-21 (3/350 cells) was confirmed in the mother in one of the families but not in the other. However, two of the relatives (first degree) were confirmed to have Down syndrome. The recurrence risk for trisomy-21 based on livebirth and prenatal diagnosis data were estimated at 1 to 2 percent for couples 35 years old or younger at the time of conception. However, such risk estimates are not available for recurrence of simple trisomy-21 to a particular couple (<35 years). Clustering of trisomy-21, trisomy-18 has been reported among Bedouins. The possibility of cryptic parental mosaicism as well as <<genetic predisposition>> should be considered in genetic counselling of families with sibships of regular trisomy-21 or other aneuploidy.