Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common inherited form of mental retardation. Symptoms, which are typically milder in females than in males, include behavioral and/or emotional problems (including autistic-like features) and varying degrees of mental impairment. Some affected males also have subtle physical abnormalities.

Fragile X Syndrome is an X-linked condition caused by a mutation on the FMR1 gene on the X chromosome. It is usually inherited from a mother who is a carrier of the condition.* Males who inherit the Fragile-X mutation are generally affected, though the severity of their symptoms can vary. About half of females who inherit the full mutation are affected.

FXS occurs in all ethnic groups, affecting an estimated 1 in 4000 males and 1 in 6000 females. Approximately 1 in 100 to 250 women in the general population is a Fragile X carrier.

*Fragile X inheritance is complicated. The FMR1 mutation involves a region of repeating DNA bases on the gene. A FMR1 gene with 55-199 repeats is said to have a “premutation” and a gene with 200 or more repeats is said to have a “full mutation.” Premutations passed on in an egg may or may not develop into full mutations.

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