Phenylalanine metabolism and intellectual functioning among carriers of phenylketonuria and hyperphenylalaninaemia.

All members of 63 families who had phenylketonuric or hyperphenylalaninaemic offspring received a phenylalanine tolerance test and an individual intelligence test. Parent carriers (heterozygotes, n=115) displayed a significant relationship (canonical correlation R=0.75, P is less than 0.05) between their ability to metabolise phenylalanine and their intellectual strengths and weaknesses. Mothers of hyperphenylalaniaemic children did not show this relationship. The number of carrier children (n=40) was too small for multivariate or sex analysis, but significant bivariate correlations were found for this group.