Variable expression of the fragile X syndrome in heterozygous females of normal intelligence.

The cognitive, linguistic and memory functions of non-retarded fragile X heterozygotes and matched controls were examined by a detailed protocol of neuropsychological measures in order to determine the incidence of specific learning disabilities in this subgroup, and to compare their performance profiles with those of control subjects (mothers of Down syndrome sons). There were no group differences in psychometric intelligence, but the heterozygotes scored significantly lower than controls on most academic achievement tests, as well as on language-based neuropsychological measures. By neuropsychological criteria, 8 of the 15 heterozygotes, but only one of the controls, exhibited performance profiles consistent with a diagnosis of specific learning disability, and similar to cognitive profiles commonly seen in individuals from the general population with developmental dyslexia.