Fragile X syndrome involves a genetic abnormality of the FRM1 gene located on the long arm of the X (sex-determining) chromosome. Although this gene normally occurs as multiple copies, in cases of Fragile X, the code is repeated in true excess. The presence of too many copies of the gene results in developmental abnormalities for the affected infant. Some family members can be carriers of Fragile X but have no symptoms at all. In other words, family members may have excessive repeats of the gene but not have enough of them to produce mental retardation. Fewer repeats of the gene mean fewer problems.
Fragile X is the most common genetic cause of mental retardation in males, who tend to be affected more severely than females who have the syndrome. Since females have two X chromosomes, the disorder is less pronounced in girls. So long as females have a working copy of the FRM1 gene, the abnormal copy is not expressed. Even so, Fragile X does remain a significant cause of mental retardation in females as well as males.
Physical characteristics of Fragile X syndrome include an elongated face and large ears. Individuals with Fragile X are generally large in stature, but with poor muscle tone. Their foreheads tend to be larger than normal and their jaws have a pronounced shape. Males with the syndrome tend to have abnormally large testicles during their adolescent years.
Behavioral features characteristic of Fragile X include a tendency of affected individuals to avoid making eye contact, to engage in odd, stereotyped movements, and to display poor social development. Other behavioral traits are hyperactivity, tremors, and poor coordination. They tend to be withdrawn and shy. Similar behavioral symptoms are also characteristic of autism, and in some cases of Fragile X, the diagnosis of autism is appropriate.
There are three subtypes of Fragile X syndrome. The first is obvious and expressed at birth. It results in learning disabilities and more significant cognitive deficits. This form is closely tied to autism and autistic traits, specifically social avoidance, stereotyped movements, and severe delays in speech and language development.
A second type of Fragile X has a significant negative impact on the individual's sense of balance. These individuals also have tremors and memory deficits. The Fragile X-Associated Tremor Ataxia syndrome (FXTAS) is a progressive condition found in some Fragile X males as they age. Fragile X-related Premature Ovarian Failure (POF) is exclusive to females, and involves problems with ovarian functioning leading to infertility and early menopause in females who have the disorder.
