How do people get hemophilia? (Level 2)

Outwardly, you appear to be a solid mass of skin, muscles and bones. Under a microscope, however, your body is made up of trillions of individual cells, that have an intricate life of their own. A cell is like a living building block; the smallest unit of living tissue. All human life starts as a single cell inside the mother. From the moment that the egg from the mother joined with the sperm from the father to create a new life, it was already determined that the one resulting cell would grow to be a boy or girl with or without hemophilia.

How could one cell determine all this? The answer lies in your genes. Inside that one cell are all the chemical instructions needed to direct the cell to make copies of itself and then divide in two. By dividing in two repeatedly, the cell reproduces itself until there are millions, then trillions of cells. Genes direct how all the copying and dividing will be done. Genes direct some cells to become heart cells, others to grow to be brain cells, and still others to grow to be teeth, the stomach lining, or your fingernails. Genes also direct your bodily functions: how your heart will work; perhaps how quickly you will be able to learn, or run; how your blood will clot.

The genes of some children do not have the correct directions for blood clotting. Children with hemophilia inherit these genes from their parents. Why do some children in a family inherit these genes, yet their brothers or sisters sometimes don't?

The answer to this question lies in the chromosomes. The genes are wrapped into 23 pairs of chromosomes, found in every single cell in your body. On one pair of chromosomes only lies the gene with the incorrect directions for blood clotting, which causes hemophilia. It lies on the same chromosomes used to determine if you will be a boy or a girl. These are called the sex chromosomes. This is why hemophilia is "sex linked" and why usually only boys get hemophilia. Follow closely now!

Inside the mother's egg are 23 chromosomes, or half of the mother's genetic characteristics. Inside the father's sperm are 23 chromosomes, half of the father's genetic characteristics. When the egg and sperm are joined, they give the new cell 46 chromosomes, arranged in 23 pairs, exactly what is needed to begin growth from a cell to a baby.

Will this new life be a boy or a girl? The last pair of the 23 pairs will determine this. Under a microscope, the sex chromosomes of a girl look like "XX." The sex chromosomes of a boy look like "XY." Whether it's a boy or a girl, you see that there is always at least one X chromosome present. The mother's egg always carries an X chromosome. But the father's sperm can carry either an X or a Y chromosome. When the egg and sperm meet and begin a new life, the egg already has an X, but the sperm may give an X to make a girl or a Y to make a boy. It is the father who always determines the sex of the baby!

Although the father's chromosomes determine the sex of the baby, the mother's chromosomes determine whether the baby will have hemophilia. The gene with the scrambled directions for blood clotting (hemophilia) is found only on the X chromosome. If the mother is a carrier of hemophilia, then half of all her X chromosomes have on them the scrambled instructions for blood clotting. The other half has clear directions for blood clotting. The clear directions, even if they are only on half her chromosomes, are enough to allow the mother to have normal blood clotting. This is why she does not have hemophilia.

But she is a carrier, so her son might receive from her the X chromosome with the scrambled directions. This is why a mother who is a carrier has a 50% chance of having a son with hemophilia. She might donate an egg with either an X chromosome affected by hemophilia or one not affected by hemophilia. This is also why her daughter might be born a carrier. The daughter might receive an affected X from her mother, but an unaffected X from her father (whose X chromosomes are all unaffected). She would not have hemophilia because the X chromosome from her father would still contain the clear directions for blood clotting.

When a boy with hemophilia, who has an affected X chromosome, grows up and has children, he will give either his X or his Y chromosome to each of his children and will determine their sex. If he gives an X chromosome to create a girl, it will have to be the one with scrambled directions for blood clotting, because, having hemophilia, all of his X chromosomes are affected. All his daughters will be carriers.