Fragile X syndrome is an inherited disorder associated with mental retardation and a particular appearance. It is caused by an error in a small piece of the DNA (genetic blueprint) for the FMR1 gene. This gene is found on the X chromosome, one of the two chromosomes (X and Y) that determine gender.
In people with fragile X, a particular section of the DNA code — CGG — is "fragile" and gets repeated an unusually large number of times, compared with the normal 5 to 50 times. As a result, the FMR1 gene is not expressed (turned on), and the body cannot produce FMR1 protein, which is related to nerve function.
Depending on the number of CGG repetitions, fragile X can have a variety of physical, intellectual and behavioral effects. People with more repetitions tend to have more severe symptoms. For example, people with 200 or more CGG repetitions are described as having a full fragile X mutation and usually develop many of the symptoms of fragile X syndrome, including mental retardation. People with 59 to 200 CGG repetitions are described as having a fragile X premutation and may not show any obvious signs or symptoms of fragile X syndrome. The number of CGG repetitions can increase when the gene is passed from generation to generation, increasing the chance of developing the full fragile X mutation.
Fragile X syndrome is seen in approximately 1 in 4,000 to 6,000 males and 1 in 8,000 to 9,000 females. However, many more people carry the fragile X premutation, but show no signs or symptoms of the syndrome. Fragile X syndrome is the most common inherited cause of mental retardation in males.