Phenylketonuria (PKU) is a genetic disorder, that is, one that depends on inherited factors and which can affect more than one member of a family.

Every new human being results from the fertilisation of an egg by a sperm. The characteristics of the father are in each individual sperm, and those of the mother are in each egg. Each egg and sperm contain thousands of tiny particles called genes. Each gene represents a characteristic of the parent which the child inherits.

When an egg is fertilized by a sperm the father's genes are added to the mother's and the fertilized egg (the new being) thus has two sets of genes. In fact, there is a pair of genes for almost every separate characteristic. The genes from the mother and father are paired, and these pairs are reproduced in every cell of the body of the new person.

Like everything else, genes are sometimes faulty and may not work properly. For most chemical (metabolic) reactions in the body only one of the pair of genes needs to be functioning fully. Thus having one faulty gene makes no noticeable effect. But if both genes of a pair are faulty problems can arise. We all have a number of these faulty genes.

A person who possesses a faulty gene without being affected by it is sometimes referred to as a 'carrier'. The gene is said to be recessive. As stated above, everyone is a carrier of some faulty (recessive) genes.

It two people with exactly the same faulty gene have children, problems can occur because some of the children could inherit the faulty gene from both parents. A person inheriting the double dose of a faulty gene is likely to have an abnormality.

PKU results from a double dose of a faulty (recessive) PKU gene. In fact about one person in 50 to 60 in our population has a faulty (recessive) PKU gene hidden away among thousands of genes they possess and yet only one person in 10,000 to 14,000 Australians has the disorder we call PKU.

The particular gene that is faulty in PKU is one that controls the amount of the enzyme phenylalanine hydroxylase. People with PKU are those who have received one faulty PKU gene from each parent. Because their cells only contain two faulty PKU genes and no normal ones they are unable to make the enzyme in normal amounts. This has a major effect on the way phenylalanine can be used in the body.

Some (but not all) of the brothers and sisters of those who have PKU will inherit a single faulty PKU gene from one or other parent. This does not matter. Since the faulty gene is matched by a normal PKU gene they will not have PKU.

1. If a couple already has one child or more with PKU, and neither parent has PKU , the chance with each subsequent pregnancy that the new baby will have PKU is 1 in 4.

2. If a person with PKU has a child with a partner who is a carrier of the faulty PKU gene then the new baby will either have PKU or be a carrier of the faulty PKU gene. 

3. If a person with PKU has a child with a partner who does not have PKU, the chance of their child having PKU is very low, about 1 in 100 to 120.

4. If two people with PKU have children, all the children will have PKU.

Those who have PKU, or who are likely to possess the faulty gene for PKU (brothers and sisters of a person with PKU), would probably feel more comfortable about having children if there was a way of testing their partner for the presence or absence of the faulty gene. Phenylalanine load test give an answer in 80 - 90% of cases. Some families know the mutations that cause PKU in their children. If that is the case other family members can have a DNA test, after talking with a geneticist or genetic counsellor. Anyone wanting more information should discuss this with their PKU clinic team, or an expert in genetic counselling.

In a family who already has one or more children with PKU it is possible to make the diagnosis in the foetus before birth. Should you wish to know more as you PKU clinic doctor.

Remember PKU can be satisfactorily treated with a low phenylalanine diet started soon after birth.