MENTAL-RETARDATION - GENETIC FINDINGS, CLINICAL IMPLICATIONS AND RESEARCH AGENDA

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Will i and Angelman syndromes, but there have also been advances in our und erstanding of the pathogenesis of Down syndrome and phenylketonuria. G enetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of m ild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild ret ardation with normal variation, is a misleading oversimplication. With in the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but t hat their interplay with environmental factors, which are also importa nt, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an importa nt research area in the future.