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. Read more…