Cytogenetic abnormalities including the marker X chromosome in patients with severe mental retardation.

Cytogenetic investigations of mentally retarded patients at the care and rehabilitation centres of the Department of Health and Welfare and at special schools have been in progress since 1972. A total of 2533 cytogenetic investigations have been carried out. A chromosomal abnormality was found in 20,6% of individuals investigated. Next to Down syndrome the Martin-Bell syndrome with the marker X chromosome occurred most frequently. This syndrome is found in the White, Black, Indian and Coloured population groups. In 15 families that have been identified, 56 males and 35 females (carriers) have the marker X chromosome. The importance and role of diagnostic/genetic investigations in mentally retarded individuals is outlined.