Phenylketonuria (PKU) is an autosomal recessive disorder in which an affected individual is unable to metabolise the amino acid phenylalanine - VCE - SSCE Biology - Question 3 - 2004 - Paper 1

The metabolic pathway indicates that phenylalanine hydroxylase is crucial for converting phenylalanine into tyrosine. Tyrosine is subsequently used to produce pigment, which is responsible for hair and skin color. In individuals with PKU, the absence of phenylalanine hydroxylase leads to an accumulation of phenylalanine. Since phenylalanine cannot be converted into tyrosine, there is insufficient tyrosine available in the body. Consequently, the reduced levels of tyrosine result in lower production of pigment, leading to lighter hair and skin color compared to non-affected siblings.