Magazine article The Exceptional Parent

Fragile X Syndrome

Magazine article The Exceptional Parent

Fragile X Syndrome

Article excerpt

Q Our son s seven years old. We know he is mentally retarded and, according to his pediatrician, he has some features of fragile X syndrome. He has had a number of diagnostic tests, including chromosome tests. All have been "negative." Our family has no history of mental retardation or any other genetic diseases. We do not want our son to go through any unnecessary testing. Do you have any suggestions? If he does have fragile X, is there anything else that can be done?

A Fragile X syndrome is the most common cause of inherited mental retardation. It occurs in approximately one in 1200 males and one in 2500 females. Fragile X syndrome is called an "X-linked disorder" because the FMlR-1 gene, the gene responsible for fragile X syndrome, is located on the X chromosome (one of the sex chromosomes; the other is the Y chromosome which only males carry). A woman who carries the gene for an X-linked condition has a 50-50 chance of passing it to either a son or daughter, since she contributes one of her two X chromosomes to each child. A man with the same X-linked gene passes it to all his daughters (they all receive his X chromosome) and none of his sons (they all receive his Y chromosome).

The severity of fragile X syndrome in an affected individual is usually related to the size of the DNA mutation within the FMR-1 gene. Some intellectually normal people have a fragile X "premutation," or small mutation. These premutations are unstable and can expand in size when passed from mother to child If the premutation expands to the size of a full mutation, the FMR-1 gene "turns off" and does not work properly. Of the females who carry a fragile X mutation, approximately 33 percent will show some symptoms; in males with the mutation gene, roughly 80 percent will have some degree of learning and behavioral problems.

Common physical characteristics of fragile X syndrome may include a long, narrow face; prominent fore head; prominent, square chin; large ears; a flattened nasal bridge; a high, arched palate (roof of mouth); a relatively large head and large testicles. These facial features tend to be more noticeable in adults than in children, and in males more often than females. Many children with fragile X who are younger than six do not differ greatly in appearance from other children. …

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