How PKU genes move through a family

In this scenario, both parents are healthy carriers for PKU. They have a 1 in 4 (25%) chance with each pregnancy of having a baby with PKU, a 1 in 2 chance (50%) of having a baby who is a healthy carrier (like them), and a 1 in 4 (25%) chance of having a baby who does not have PKU and is not a carrier. Because these chances are the same with each pregnancy, it is possible for two carriers to have all kids with PKU, no kids with PKU, or a mix.

Here one parent has PKU (2 non-working PKU genes). The other parent does not have PKU and is not a carrier. All of their children will be carriers, but will not have PKU.

Here one parent has PKU and the other parent is a carrier for PKU. There is a 1 in 2 (50%) chance that a baby will have PKU, and a 50% chance that a baby will be a healthy carrier.

You might ask, ok, I have PKU, how likely is it that my husband / wife/ boyfriend/ girlfriend will be a carrier for PKU? If a person has no family members with PKU, their chance for being a carrier is approximately 1 in 50 (2%). So if we know that you have PKU, and we don't know anything about the other potential parent, we would say that there is a 1 in 100 (1%) chance that your child would have PKU.

If two people with PKU have children, all of their kids will have PKU.