The simple answer is that your baby has inherited PKU from both of you. Your immediate response may be that neither you nor your partner have any knowledge of anyone in your families having PKU. In order that you can understand how PKU is inherited, we have to explain the way in which characteristics are passed on from parents to children.
We all know that children tend to have features such as the colour of hair and eyes which make them resemble one or other parent. All of our
inherited characteristics are controlled by the store of information contained within the cells of our bodies. These information stores are called genes and these genes have been copied from the set of father’s genes contained in the sperm and the set of mother’s genes contained in the ovum or egg. Therefore, from the original sperm and ovum which made your baby there will be a father’s gene and a mother’s gene controlling each inherited characteristic. Usually we need only one gene to control one activity in the cell and in the body, so that if a gene inherited from one parent is defective, the gene from the other parent will allow the cell and body to work normally.
In PKU both genes which control the production of the enzyme phenylalanine hydroxylase are faulty ie, the child with PKU has inherited one
faulty gene from the father and one from the mother.