Our DNA contains the genetic instructions for the development of most inherited traits. Among the genes and chromosomes we inherit from our parents are two sex chromosomes, labelled X and Y. In terms of genetics, haemophilia is known as a sex-linked disorder because the genes responsible for producing Factor VIII and IX are situated on X chromosome. Females inherit two X chromosomes, one from the mother and one from the father. Males inherit one X chromosome from the mother and one Y chromosome from the father.
Since a man has only one X chromosome, if it has the haemophilia gene they will have haemophilia. All daughters of a man with haemophilia will carry the haemophilia gene, the X chromosome they received from their father, which they can then pass on to their children. The sons of a man with haemophilia will not be affected unless their mother also carries the gene for haemophilia (very rare).
Since women have two X chromosomes they are usually less severely affected by haemophilia than males. It is unlikely that both a women’s X chromosomes will contain the haemophilia gene (unless both parents are affected). They can still pass on the gene and are often affected by milder bleeding symptoms. For each child of a women with the haemophilia gene, there is a 50% chance that a son will have haemophilia and a 50% chance that a daughter will carry the gene. In some cases haemophilia is "hidden" for many generations if no affected male children are born. In such cases, the gene for haemophilia is carried through several generations of females who, because they have a second X chromosome that is unaffected, do not suffer from the disease themselves. Most mothers of children with haemophilia have fathers, grandfathers, brothers or other male relatives on the maternal side of their families who were born with haemophilia.