Rationale

Haemophilia is a genetic disorder, which is due to the mutation (change in structure) of the gene responsible for producing Factor VIII or Factor IX. Haemophilia is also typically hereditary, meaning it is passed down from one generation to the next through these "defective" (mutated) genes. Specifically, the male inherits the mutated gene from his mother. This is because the gene is located on chromosome X of the gender. What exactly does this mean?

Genes are located in chromosomes. Individuals inherit one chromosome from their father and one chromosome from their mother.

The gender of a person is determined by the sex chromosomes. These are the X and Y chromosomes. Females have two X chromosomes (ΧΧ) while males have an X chromosome and a Y chromosome (ΧΥ).

Thus, each male has only one X chromosome. If the boy then inherits from his mother the X chromosome in which the haemophilia gene is located, then this boy will develop haemophilia.

Females have two X chromosomes. If a girl inherits an X chromosome with the mutated gene, she will not develop haemophilia, as long as the second X chromosome is normal. This girl will, however, be a carrier of haemophilia, which means that there is a 50% chance to pass the chromosome with the mutated gene, on to her son. On the other hand, if she passes this chromosome on to her daughter, the daughter will also be a carrier of haemophilia.

Therefore, although females who are carriers do not develop haemophilia, they can pass it on to their son or daughter. What applies to males who have haemophilia? If a male with haemophilia had a daughter, she will have inherited the X chromosome with the mutated gene and will be a carrier of haemophilia.