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During the past two decades, progress in understanding some genetic defects responsible for many physical and mental debilities has been impressive. It is now recognized, for example, that mental retardation occurs in appreciable numbers of humans with numerical and structural chromosome disorders (Down syndrome, cri-du-chat syndrome, etc). In other individuals, a single gene abnormality such as phenolketonuria has been identified at birth, and treatment can now be initiated to preserve life and reduce the debility. With other diseases, such as mucopolysaccharide storage disease, a specific biochemical defect may be identified, but satisfactory treatment is not yet available. In still other abnormalities, the causes are understood poorly and no treatment exists. It is now even possible to identify several single gene diseases in the 16- to 20-week-old fetus by means of biochemical studies on cells obtained by amniocentesis; in addition, all gross chromosome disorders can be recognized prenatally.
Howard RO. The Prevention of Genetic Disease and Mental Retardation. Arch Ophthalmol. 1976;94(4):693. doi:10.1001/archopht.1976.03910030347031