Intracellular phenylalanine and tyrosine concentrations in 19 heterozygotes for phenylketonuria (PKU) and 26 normals. Do the higher values in heterozygotes explain their lowered intellectual level?

Intracellular phenylalanine and tyrosine was determined in lymphocytes of 10 heterozygotes (parents) for PKU and in 26 randomly collected apparently normal persons. In cells from the heterozygotes the concentrations of both phenylalanine and tyrosine were higher than in those from the normals, the difference being statistically highly significant. It is argued that this could be responsible for the slight, though statistically significant, intellectual inferiority of heterozygotes for PKU.