Phenylketonuria - Causes of phenylketonuria

Phenylketonuria (PKU) is a genetic condition that's passed to a baby by its parents.

Genetic mutation

PKU is caused by a genetic mutation (change to the DNA) in the human phenylalanine hydroxylase (PAH) gene.

There are many different types of mutations that can affect the PAH gene, all of which result in the body not producing a functioning PAH enzyme.

Enzymes are proteins that speed up and control chemical reactions. Lack of the PAH enzyme means that a chemical called phenylalanine, which is found in some foods, can't be processed properly by the body.

Autosomal recessive mutation

All the genes in your body come in pairs. You receive one half of the pair from your mother and the other half from your father.

The mutation that causes PKU is an autosomal recessive mutation. This means you need to receive two mutated copies of the gene (one from your mother and one from your father) to develop the condition.

You won't develop PKU if you only receive one mutated copy of the gene from one of your parents, but you'll carry the mutated gene.

If you're a carrier of the mutated gene and you conceive a baby with a partner who's also a carrier, there is a:

• 1 in 4 chance that your baby will receive a pair of normal genes
• 1 in 2 chance that your baby will receive one normal gene and one mutated gene and become a carrier of PKU without developing symptoms
• 1 in 4 chance that your baby will receive a pair of mutated genes and develop PKU