Question

Haemophilia is a case of recessive expression of genes. In a case of two sons of a couple , one is haemophilic. What is the probability of inheritance of haemophilia in their daughter?

Answer 1

Since one son is haemophilic and the other is not the mother must be a carrier of the gene. If the father is normal then the daughter will be a carrier of the gene. If however, the father is haemophilic then the daughter may be either haemophilic (if the affected gene is inherited from the mother) or a carrier (if the unaffected gene is inherited from the mother) and the probability of both will be 1/2.