Chromosome abnormalities in South African mental retardates.

Standard and differential staining techniques were employed in this cytogenetic study of mentally retarded Whites at the Umgeni Waterfall Institution. All of the 512 patients were karyotyped and 57 were found to have chromosome abnormalities. Of these, 42 had trisomy-21; there were 3 subjects with 5p deletion (cri-du-chat) syndrome, 3 had supernumerary small marker chromosomes, and 2 had complex structural rearrangements. Gonosomal aneuploidies were less common than the autosomal defects and only 2 poly-X males and 1 poly-X female were identified. Long Y chromosomes were found in 11 males and 4 others had deleted Y chromosomes. One abnormal chromosome, a deletion of the terminal region of 11q, was missed in unbanded karyotypes. Banding is essential to the identification of structurally abnormal chromosomes.