Genes, Chromosomes, and PKU

The cell is the building block of living things. Our bodies are made up of millions of cells. Inside each cell, in a special compartment called the nucleus, is a set of packages called chromosomes. Chromosomes contain thousands of genes, the recipes for all the chemicals that our cells need to grow and stay healthy. Genes also determine features like eye color, height, and hair color. We inherit our genes from our parents. This is why children resemble their parents. There are 46 total chromosomes in each cell. They come in 23 pairs, numbered by size. We have a pair of chromosome number ones, a pair of number twos, and so on. We get one chromosome from each pair from our mom (through her egg) and one from each pair from our dad (through sperm). Because our chromosomes come in pairs, so do our genes. Genes can sometimes contain mistakes so that the cell can't read them properly. We can call this a "non-working gene". The gene that is important in PKU is located on the chromosome pair number 12. People with PKU have two non-working copies of this PKU gene. One copy they inherited from mom, and the other from dad. Usually, parents don't even know they are carriers of PKU until they have a baby who has PKU. A "carrier" is a person who has one non-working gene and one working gene. They are healthy, but can pass the non-working gene on to their kids. The way that PKU is passed through families is called an autosomal recessive trait. This means that you need to inherit two non-working PKU genes to have PKU, and that males and females can both have PKU. Remember that each parent passes on one gene from each pair to the sperm or egg. It is completely random which chromosome from each pair is passed to the baby through sperm and egg. Because of this, with each pregnancy, there are usually a few different possible outcomes, depending on how many non-working genes that parents have.

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.

	How can I learn more?
	Talk to a Genetic Counselor. A Genetic Counselor is a person who works with families or individuals who have questions or concerns about genetic conditions.