Down's Syndrome Transmitted Through Maternal Mosaicism

Summary Two families are reported in which Down's syndrome was transmitted through phenotypi‐cally normal mosaic mothers. In the first family the mother had normal/G‐trisomy mosaic, and on two successive pregnancies at the age of 22 and 24 years she gave birth to two sibs with regular G2i‐trisomy and Down's syndrome. In the other family the mother had a mosaic of normal and balanced D/G21 translocation cells, and at the age of 32 years she gave birth to a child with D/G2t translocation trisomy and Down's syndrome. Mosaic mothers carry a considerable risk of having children with Down's syndrome. Exact prediction of the risk cannot be given since the extent of gonadal involvement in the mosaicism is unknown.