Science / Medicine : Genetic Link to Retardation

A genetic discovery may pave the way for better tests to determine if an unborn baby will inherit a common form of mental retardation, French researchers said last week. They reported in the journal Nature that they have found new genetic features of Fragile X syndrome--the second most common type of inherited mental retardation after Down’s syndrome--that may help improve genetic counseling for the disorder.

In Fragile X syndrome, which affects one in every 1,250 male births, a particular site on the sex-determining X chromosome is very vulnerable to breakage. Females have two X chromosomes, making them less vulnerable to the disorder because they can also inherit a healthy X chromosome; males have an X and Y chromosome.

The researchers found that a small piece of genetic material near the fragile site on the X chromosome of affected people appears to be inactivated. All 21 fragile X males who were mentally retarded had the abnormal genetic pattern; 22 normal men and four fragile X men who were not retarded had normal patterns.

If the findings are confirmed and shown to apply to fetuses, it may be possible to devise a prenatal test that could predict whether an unborn baby with a fragile X chromosome will be mentally retarded or simply be a mentally normal carrier, the researchers said.