Daniel is 16 months old who has phenylketonuria (PKU). PKU is caused a a recessive condition that results in a mutated amino acid sequence of an enzyme called phenylalanine hydroxylase. Without this enzyme, the body cannot catabolise phenylalanine in the body, so it accumulates has a toxic effect. In the body, phenylalanine is broken down to another chemical called tyrosine that is used to make several chemicals including adrenaline. If phenylalanine cannot be broken down, there is no tyrosine and therefore no adrenaline. The chance of future children inheriting PKU is of great concern to Daniel’s parents (both unaffected) and they are hesitant to have another child – so they also seek genetic counselling at the clinic.What advice would you give them if you are doing the genetic counselling? 

Phenylketonuria is a recessive condition so Daniel must have received one copy of the disease-causing gene from both his parents. Only a child with 2 copies of the gene will have the disease. His parents may be unaffected because they have only one copy of the gene each-in this case they are known as carriers. In theory there is a 25% chance of 2 carriers producing a child with PKU. However, there is no guarantee that that will occur. Each time a child is conceived it receives genes from the egg and sperm, and the genes that it receives a not influenced in any way by the genes that were passed to previously conceived children. These 2 carriers could go on to have 10 children, all or none of whom could have PKU. There is no way of predicting whether an embryo will be homozygotes for the PKU gene and therefore have PKU once it is born. Post conception genetic testing could identify a homozygotes embryo thus putting the parents in the position where they need to decide what to do about the pregnancy. Alternatively, artificial reproduction techniques could be used to identify eggs that don't have the disease gene and those eggs could be used in procedures like in vitro fertilisation to produce a child who did not have PKU.