Dictionary of Human Evolution and Biology

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Fragile X Syndrome

Pervasive developmental disorder that results in profound mental retardation, a consistent facies, and large testicles in males. FRAX is the most common heritable syndrome with symptoms of mental impairment and behavioral change. Transmitted as an X-linked recessive, it affects hemizygous males far more frequently than females; females are usually heterozygotes and exhibit milder symptoms, as they possess only one working copy of the gene. The syndrome is caused by a mutation in the fragile X mental retardation protein (FMR1, an X-linked recessive), which amplifies a three nucleotide repeat up to 200 times (see anticipation), so that the FMR1 protein is not produced. Normally, the FMR1 protein functions to bind RNA. Female carriers of the disorder have pre-mutations that can be identified at the molecular level. FRAX is not the same clinical entity as mental retardation, X-linked nonspecific.

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