Haemophilia is a genetic blood clotting disorder which in simple terms means anybody who has the disease can't stop bleeding should they be cut. They also tend to bruise more easily, due to internal bleeding.
Haemophilia, is a condition which is inherited from parents. Generally only males suffer from haemophilia, but both males and females can pass on the genetic defect.
The genes in our cells are arranged in long chains, called "chromosomes". Two chromosomes; in particular called "X" and "Y" determine the haemophilia condition. The "X" chromosome is indirectly responsible for the body to produce clotting factors.
Females have two "X" chromosomes whereas males have an "X" and a "Y" chromosome. This difference in genetic makeup accounts for females being carriers (and to a lesser extent, sufferers) and males being sufferers.
A father can pass either an "X" or a "Y" chromosome to his children whereas a mother can pass only an "X" chromosome to her children.
An "X" chromosome from a mother together with an "X" chromosome from the father will mean the child is a female. Whereas an "X" chromosome from a mother together with a "Y" chromosome from the father will mean the child is a male.
If an "X" chromosome in a father is damaged (i.e haemophiliac) it means that all his daughters must be carriers because they would have received the damaged "X" chromosome, his sons will not be affected as only the "Y" chromosome can be passed to males.
On the other hand, if a mother is a carrier there is a 50:50 probability of her passing her faulty "X" chromosome on to her children. Only her sons may have haemophilia (sufferers) and only her daughters may be carriers.
The X chromosome is responsible for supplying information to the body to produce clotting factors.
In the case of females, they have the two X chromosomes, so the dominant non-defective X chromosome will supply information to the body to produce clotting factors to compensate for a defective X chromosome.
In the case of males, as the Y chromosome does not supply information to the body on how to produce clotting factors they will be sufferers if they inherit a defective X chromosome.
In summary:
In the case where a mother is not a carrier but the father has haemophilia, he will pass the condition onto all his daughters, but not his sons. They will inherit their mother's non-defective dominant X chromosome.
In the case where the father does not suffer haemophilia but the mother carries defective X chromosome, there is a 50/50 chance of each son or daughter inheriting the defective gene.
The likelihood of inheriting the condition from a father suffering the condition is definite for female children and nil for male children (unless the mother is a carrier).
The chance of inheriting the condition from a mother carrying the condition is 50/50 for both male and female children.
The situation is more complicated if both the mother and father carry a faulty "X" chromosome.
If a haemophiliac man has a daughter with a carrier woman, there is a 50/50 chance the daughter will be a haemophiliac. If this happened, and the haemophiliac woman had a
daughter to a haemophiliac man, the chance of the daughter being a haemophiliac increases to 100%. The likelihood of these situations occurring is small (particularly the latter situation) but it can and does happen.
There is a certain amount of speculation on which members are Queen Victoria's family were carriers or sufferers. David Duff's book "Hessian Tapestry" page 167 quotes Queen Victoria's daughter Victoria being a carrier.