In Reply.—Dr Bowers has thoughtfully described two mechanisms whereby boys with fra(X) syndrome could present with normal intelligence. She has discussed a genetic background of high intelligence and maturational acceleration so that the chronologic age is appreciably below biologic age and the IQ is thus inflated. We have not seen evidence for accelerated maturation in the growth percentiles of boys with fra(X) syndrome, although I am not aware of bone age analyses in patients with the syndrome. In our experience very intelligent parents often have high-functioning children with fra(X) syndrome, which suggests the influence of a genetic background of high intelligence; however, this is not always the case. Instead, a third mechanism most probably exists: variability of penetrance or effect of the fra(X) gene itself.
Variable penetrance is seen in heterozygous girls, with a frequency of approximately one third being mentally retarded1 and the rest being either learning
HAGERMAN RJ. Fragile X Syndrome and Learning Disabilities-Reply. Am J Dis Child. 1986;140(4):327-328. doi:10.1001/archpedi.1986.02140180061017