A relatively common genetically inherited abnormality of the X chromosome which results in mental retardation. Since it is an X-linked trait, males more often have it expressed in their phenotypes. Most fragile-X males have large testes, big ears, narrow faces, and sensory integration dysfunctions that result in learning disabilities. It is likely to occur 1 in 1000 births. Approximately 1 in 700 females are carriers of the gene for this trait.