Severe mental retardation in six generations of a large South African family carrying a translocation t(6;10)(q27;q25.2).

Partial monosomy 10q25.2----qter, detected in a newborn baby with multiple congenital abnormalities, was found to be derived from a balanced maternal translocation t(6;10)(q27;q25.2). The pedigree of six generations of the family is presented. In an extensive cytogenetic study of this family, the chromosome complements of 57 subjects, potentially capable of carrying some form of this translocation, were analysed. A total of 14 male carriers (four obligatory) and 14 female carriers (three obligatory) of this translocation was found. Partial trisomy 10q25.2----qter, associated with severe mental retardation, occurred in nine cases, eight males and one female. Two of these eight males were detected prenatally and subsequently therapeutically aborted. The phenotypes of the family members with partial trisomy 10q25.2----qter are compared to each other and to those reported in publications. No further cases of partial monosomy 10q25.2----qter were encountered. A review of published reports of partial monosomy and partial trisomy 10qter is given. The apparent absence of infertility, the occurrence of many first trimester miscarriages, and the marked sex ratio are discussed.