Phenylketonuria (PKU) is an example of a genetic disorder which affects the following metabolic pathway - Scottish Highers Human Biology - Question 2 - 2017

The substitution mutation may change one amino acid in the protein, leading to the production of an enzyme with a different amino acid sequence. This may affect the active site of the enzyme, resulting in reduced or complete loss of enzyme function.

PKU causes a build-up of phenylalanine because enzyme 1, which is supposed to convert phenylalanine to tyrosine, is faulty and does not function effectively. As a result, phenylalanine accumulates in the body.

Individuals with PKU can still produce melanin because melanin synthesis involves tyrosine, which can be derived from other metabolic pathways. Although phenylalanine cannot be converted into tyrosine, if tyrosine is consumed directly in the diet, melanin production can still occur.

The term used to describe this type of diagnostic testing is 'post-natal screening.'

Brain damage can be prevented in babies diagnosed with PKU by implementing a restricted diet low in phenylalanine. This dietary control minimizes the accumulation of phenylalanine in the body.

If both parents are carriers of the autosomal recessive allele for PKU, there is a 25% chance (or 1 in 4) that their next child will inherit PKU. This can be calculated using a Punnett square.