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Fragile X syndrome is arguably the single most common inherited cause of mental impairment. Current estimates of its prevalence vary, but some experts believe that fragile X affects at least 1 in 1000 men and women of all races and ethnic groups. More conservative estimates put the frequency at 1 in 1500 men and 1 in 2500 women. The discrepancy in these numbers is due to the fact that large-scale population studies of the incidence of fragile X have not yet been.
Boys are typically more severely affected than girls. Although most boys with fragile X syndrome have MR, only one third to one half of girls have significant intellectual impairment; the rest have either normal IQ or learning disabilities. Emotional and behavioral problems are common.
Among persons institutionalized because of mental retardation, a 30 to 50 fold excess of men is well documented. Family studies often reveal affected male siblings and maternal uncles, leaving no doubt that a group of X-linked mutant genes causes MR.
This at least in part accounts for the excess of retarded men. The clinical features include large testes, especially after puberty, large protuberant ears, and prominent chin and forehead. Accurate pre- and postnatal detection will be provided when the fragile X gene is cloned and characterized. The fragile X syndrome is of major importance because, next to Down syndrome, it is the most common cause of MR.
There are excellent DNA analyses to determine the presence of fragile X.