What is the risk of a child inheriting haemophilia A, when both parents are unaffected, but the mothers father suffered from the condition?

If the mother’s father had haemophilia the mother must carry the allele for the condition, as the mother must receive an X chromosome from her father. Therefore, the X chromosome received from her mother must not have the allele for haemophilia, otherwise she would present with the condition. However, from the question we know she is unaffected. Thus we can describe the mother as heterozygous with respect to the haemophilia allele. Males only have one X chromosome and if the haemophilia allele is present they will always present with the disease, as the allele is recessive and is found on the X chromosome. For this reason the father must not have the haemophilia allele at all. You now need to calculate the risk of a child born to these parents inheriting haemophilia A by drawing a Punnett square. So we know that there is 1:1 ratio of females born to males. Females receive two copies of the X chromosome, one from the mother and one from the father. Therefore, any female born will not present with haemophilia. There are two possibilities for daughters either heterozygous or homozygous. Males receive one X chromosome from their mother and a Y chromosome from their father. As the mother has one x chromosome which does and 1 that doesn’t contain the faulty haemophilia allele, the risk that a male born will have an X chromosome containing the haemophilia allele is 50%. We now combine this to two possible outcomes for females. There are 4 options, two females who are not haemophiliacs, one male who is not a haemophiliac and a male who is. The risk for haemophilia must be 25% or 1 in 4 children born.