Genetic mutation
Phenylketonuria (PKU) is caused by a genetic mutation in a gene that is known as the human PAH gene. A gene is a single unit of genetic material. A genetic mutation occurs when the instructions that are carried in certain genes become ‘scrambled’. This results in some of the body’s processes not working in the normal way.
There are over 100 different types of mutation that can affect the PAH gene, all of which result in the same outcome. The body does not produce an enzyme that is needed to break down phenylalanine before reassembling it into more complex chemicals. This leads to the high levels of phenylalanine that are associated with PKU. Enzymes are proteins that speed up and control chemical reactions in 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 that you need to receive two copies of the mutated gene in order to develop the condition - one from your mother and one from your father.
You will not develop PKU if you only receive one copy of the mutated gene from one of your parents, but you will carry the mutated gene.
If you are a carrier of the mutated gene and you conceive a baby with a partner who is also a carrier there is a:
- 1 in 4 chance that the baby will receive a pair of normal genes
- 1 in 2 chance that the baby will receive one normal gene and one mutated gene and become a carrier of the mutated PAH1 gene but will not develop any symptoms.
- 1 in 4 chance that the baby will receive a pair of mutated genes and develop PKU
