Normal IQ distributions of early-treated phenylketonuric children and their unaffected siblings: failure to replicate a trimodal response or negative skew.

Abstract

PHENYLKETONURIA, an inherited metabolic disorder associated with mental retardation, has been generally considered a single gene defect. Fuller and Schuman1, however, have argued for a more complex genetic model, citing a trimodal frequency distribution of IQ scores for a sample of treated phenylketonurics (PKUs) and a negative skew in the IQ distributions of unaffected siblings as evidence for their model. We have analysed data collected by Hudson2 to test the basis of Fuller's findings. Evidence was found for neither a trimodal response to treatment of phenylketonuria nor a negative skew in the IQ distribution of their unaffected siblings. Both groups showed IQ distributions which were not significantly different from the expected, normal, curve for that sample.