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Genetics: The Basics

Hemophilia is generally considered a genetic disorder, meaning that it is caused by a gene that does not work the way it should because it is mutated, or changed from normal.

To understand how hemophilia is inherited within families, it’s helpful to learn some of the basics of genetics.

Genetics is the study of how genes are passed from one generation to the next. Every cell in our body has genes. Our genes are what make us who we are – they form our body’s blueprint. Things like eye, skin and hair colour are all determined by our genes. And we have a lot of them – about 20,000 to 25,000. Our genes contains DNA, which is a “code” that instructs the cells in our bodies to produce substances called proteins. The DNA needs to be in a specific sequence to produce the protein properly There are thousands of proteins that do different things in our bodies. Clotting factors, for example, are types of proteins.

What are chromosomes?

A chromosome is a collection of genes. There are 23 pairs of chromosomes in a human cell: one of each pair is inherited from your mother and the other from your father. So, our chromosomes determine what traits we inherit from our parents.

What are sex-linked genes?

One pair of chromosomes are the sex chromosomes: "X" and "Y." Women have two X chromosomes and men have an X and a Y chromosome. Genes that are carried on the X chromosome are called X-linked, or sex-linked genes.

The genes that control the production of Factor VIII and IX are part of the X chromosome. This is why hemophilia A and B are considered “sex-linked disorders”, which means one sex (males) is predominantly affected.

X vs. Y: The difference between girls and boys

To understand how hemophilia is inherited, we need to understand how a baby is conceived. When a child is conceived, a male sperm fertilizes a female egg. The woman's egg contains two X chromosomes, while a man's sperm has both an X and a Y chromosome. Each parent contributes equally to their new child: a father passes on either an X or a Y, while a mother passes on one of her Xs. So at conception, there are four possible ways the egg and sperm cells can combine, which determines the child’s sex:

Diagram showing how mothers pass on X chromosomes and fathers pass on an X or Y chromosome

The mother always passes on an X chromosome, so it’s the father that determines the baby’s sex:

  • If the father passes on an X chromosome, the child will be a girl (XX).
  • If the father passes on a Y chromosome, the child will be a boy (XY).

XMYTH: Boys always inherit the hemophilia gene from their mothers.

checkmarkFACT: In most cases, the gene responsible for hemophilia is passed from a mother her son at the time of conception. But, 3 out of 10 boys develop hemophilia without inheriting it from his mother and when there is NO family history.

Information Booklet on Mild Hemophilia

A booklet for people living with milder forms of hemophilia, developed by the Atlantic Hemophilia Nurses Group of the Canadian Association of Nurses in Hemophilia Care (CANHC).

Download Booklet on Mild Hemophilia

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