Genetic and biological advances help unlock mysteries of mental retardation.

Byline: Sue Goetinck Ambrose

DALLAS _ Scientists devote a lot of brainpower to figuring out how the brain works. They're also trying to solve something equally mysterious _ why, in cases of mental retardation, the brain doesn't work quite right.

The hundreds of known causes of mental retardation range from genetic abnormalities to toxic substances such as alcohol and lead. Still, for about a third of people with mental retardation _ defined as an IQ below 70 to 75 _ no cause has yet been found.

But new avenues of research based on advances in genetics, biology and cell science are helping brain scientists solve these mysteries.

In Down syndrome brains, for instance, cells that are responsible for producing nerve cells make too few of them, recent research has revealed.

In people with Fragile X syndrome, the leading inherited cause of mental retardation, nerve cell connections may not be maturing completely. Fragile X syndrome affects mostly boys.

Another significant cause of mental retardation in boys may be abnormal blood vessels in the brain. In many other cases of mental retardation, misshapen knobs that link nerve cells can dampen the passage of electrical impulses.

The new research represents a different approach to a perplexing set of conditions.

"For a long time it was just looking at…