Genetic Fundamentals
Hemophilia is 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. Like other genetic health problems, hemophilia can be passed from generation to generation. In almost all cases, the gene responsible for hemophilia is passed from a parent to the child at the time of conception.
In order to understand hemophilia, you need to know about genes and genetics. If there is a history of hemophilia in your family, you will need to know whether you can pass hemophilia on to your children, or if you or your daughter are carriers of hemophilia.
Genetics is the study of how genes are passed from one generation to the next. Each cell inside your body contains genes. There are about 40,000 genes in the human body that make up what is called the genome. These genes store information that will be passed along to your sons or daughters. The "instructions" for making proteins such as Factor VIII are found in your genes.
What Is DNA?
DNA (deoxyribonucleic acid) is a long fibre, like a hair, only thinner and longer. It is made from two strands that stick together and are slightly twisted. This is called a double-helix and it looks like a spiral staircase. The steps of this staircase contain molecules in various pairs that determine a person's genetic code.
What Are Genes?
DNA is organized into thousands of units called genes that are present in every cell in the body. Each gene contains the code needed to construct one protein. There are between 30,000 and 40,000 genes in each cell and they carry the instructions for making all the thousands of proteins that are found in a cell. (Many substances in the body are proteins, such as the blood clotting factors.) Some cells have certain genes turned "off" and others are turned "on." This is why, for example, your liver cell is different from your muscle cell, and your muscle cell is different from your skin cell. The sequence of the DNA within the gene also determines genetic traits, such as hair, eye, and skin colour, among many other things.
What Are Chromosomes?
A chromosome is a collection of genes. The genes are arranged in specific patterns and make up different types of chromosomes. Therefore, chromosomes carry our hereditary information. 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.
One pair of chromosomes are the sex chromosomes: "X" and "Y." Women have two X chromosomes and men have an X chromosome and a Y chromosome. Genes that are carried on the X chromosome are called X-linked, or sex-linked genes.
The genes that regulate the production of Factor VIII and IX are part of the X chromosome. They play an important role in how hemophilia is passed from person to person in families.
Sexual Reproduction and the Passing of Chromosomes
To understand how hemophilia is inherited, you need to understand how genes get passed on from parents to children. Every single person starts out exactly the same way: a male sperm fertilizes a female egg and a child is conceived. The woman's egg contains two X chromosomes, while a man's sperm has both an X and a Y chromosome.
When the sperm and egg cells combine their DNA to produce a brand new person, they each contribute one of their sex (X and Y) chromosomes: the man's sperm passes on either an X or a Y, while the woman's egg passes on one of its Xs. So at conception, there are four possible ways the egg and sperm cells can combine and determine the child's sex—as shown in this illustration:
Because a woman's egg ONLY has X chromosomes to give to a child, it is the man's sperm that determines the sex of a child. In other words:
- If the sperm cell that reaches the egg contains an X chromosome, the child will be a girl (because it receives XX chromosomes).
- If the sperm cell that reaches the egg contains a Y chromosome, the child will be a boy (because it receives XY chrosomes).