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) repeats, whereas the number of repeats present in the normal version of each gene vary across individuals, but does not appear to exceed 50. In this study, the association between the number of CGG repeats and IQ score was examined among 902 school age children. Separate analyses were conducted with the entire sample, among only Caucasian or only African American children, and among only boys or girls. None of the statistical models examined revealed a significant association between full scale IQ (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.
Bibliographical noteFunding Information:
This research was supported by the following grants: K02MH00142, NS35359, and R01MH50047 from the NIH. The authors wish to thank the Directors of the clinics from which the sample was obtained, including Drs. Bruce Shapiro, Ronald Walcher, and Martha Bridge Denckla; the psychologists at each of these clinics; the Johns Hopkins DNA Diagnostic Laboratory; and Research Assistants James Teisl, Nancy Lee Sonna, and Gwen Myers.
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