What is the probability of offspring (both male and female) being affected by haemophilia if both parents are unaffected?

Haemophilia is a sex-linked genetic recessive disease, meaning it is associated with a gene located on the X chromosome. As males have only one X chromosome, inheriting that chromosome will mean that they will be affected by the disease. Females however, have two X chromosomes and as haemophilia is a recessive disease, two defective X chromosomes would have to be inherited for a female to be affected. In this case, the father is unaffected, and as he has to pass on a healthy X chromosome to his daughters, the probability of female offspring being affected is 0, regardless of the genotype of the mother. Any male offspring will inherit a Y chromosome from their father and an X chromosome from their mother, but there is a possibility that the mother might be a carrier (if she has both a healthy and a defective X chromosome) even though she is unaffected. If the mother is a carrier (which we don’t know in this question), the probability of a son inheriting the defective X chromosome and being affected by haemophilia is 50%.