What causes phenylketoneuria?

The protein in our food is made up of different building blocks, one of which is phenylalanine. Too much phenylalanine in the body can interfere with brain development. People with phenylketoneuria (PKU) have no working version of a gene that makes an enzyme called phenylalanine hydroxylase, which normally breaks down excess phenylalanine in the body. Carriers of PKU only have half the normal levels of phenylalanine hydroxylase in their bodies, but they are not affected by the condition.

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Anna with her children Madeleine (centre), who has PKU, and Isobel. Madeleine's PKU is managed successfully by diet and she excels at her school work.

How is phenylketoneuria inherited?

Phenylketoneuria (PKU) affects people who inherit two copies of an altered gene from their carrier parents. Carrier parents have a one-in-four chance of having an affected child, a one-in-two chance of having a child who is an unaffected carrier and a one-in-four chance of having a child with no altered genes at all. Carriers of PKU are not affected themselves, as they have a working gene as well as an altered one.

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Inheritance of PKU.


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