Normal variation in size of the FMR1 gene is not associated with variation in intellectual performance

The fragile X mental retardation (FMR1) gene was examined as a potential single-gene candidate for variation in intelligence score, on the basis of its well established relation with mental retardation. The size of the normal FMR1 gene varies across individuals, and is measured by the number of cytosine-guanine-guanine (CGG) repeats of which it is comprised. In this study, the association between IQ score and the number of CGG repeats was examined among 917 school age children, including 455 Caucasian children and 462 African American children. Separate analyses were conducted with the entire sample, with girls and boys examined separately, and with Caucasian and African American subgroups. None of the models revealed a statistically significant association between IQ and number of CGG repeats. The results from this study provide strong evidence that variation in CGG size, among normal size FMR1 alleles, does not account for variation in intelligence.