[Genetics of mental retardation].

Mentally retarded patients can be grouped in two categories: severe forms seen in 4 of every thousand live births and mild forms that occur approximately five times more often. Approximately half of severe cases of mental retardation are genetically determined, and half of these fall under the category of X-linked mental retardation (XLMR) disease. The XLMR group is currently comprised of 105 highly varied types of retardation that can be associated with a fragile X chromosome, biochemical defect, neurologic alterations, bony dysplasia and a range of malformations. Along with such syndromes (which are specific), some 40 familial XLMR entities (nonspecific) can be identified in which mental retardation is the only sign. Cytogenetic testing to identify dysmorphic syndromes caused by chromosomal disease and molecular biology studies are indispensable for identifying the genes responsible for XLMR syndromes. Healthy carriers can also be found. It thus becomes possible to provide appropriate genetic counseling for families and to achieve a prenatal diagnosis in some cases.