Which of the following is incorrect about phenylketonuria?

Phenylketonuria (PKU) is an autosomal recessive disorder. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). 

This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation. 

The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives. 

Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.