Normal variation in size of the FMR2 gene is not associated with variation in intellectual performance

In this study, a fragile X mental retardation (FMR2) gene was examined as a potential single-gene candidate for intelligence variability. The FMR1 mutation is the most common etiology of fragile X syndrome. Although the FMR2 mutation is believed to be less common than the FMR1 mutation, both gene mutations are associated with mental retardation and learning disability. For both genes, mutations consist of over approximately 200 cytosine-guanine-guanine (CGG) repeals, whereas the number of repeals present in the normal version of each gene vary across individuals, but does nor appear to exceed 50. In this study, the association between the number of CGG repeats and IQ scole was examined among 902 school age children. Separate analyses were conducted with the entire sample, among only Caucasian or only African American children, and alnong only boys or girls. None of the statistical models examined revealed a significant association between full scale Ig (FSIQ) and the number of CGG repeats. The results from this study indicate that variation in CGG size, among normal size FMR2 alleles, is not a contributor to normal variation in intelligence.