What is the risk of hemophilia for her children? II3 in the pedigree below has two brothers with hemophilia A, a bleeding disorder that is inherited as an Xlinked recessive trait.
A. 1 in 4 for a son, close to zero for a daughter B. 1 in 2 both for sons and daughters C. 1 in 2 for a son and 1 in 4 for a daughter D. 1 in 2 for a son, close to zero for a daughter E. 1 in 4 both for sons and daughters
Use a pundit square. They kindly give the mode of inheritance as X-linked recessive. Both sons are affected. Daughter unaffected. Father unaffected. Mother unaffected. Therefore, mother must be aA and father A. Sons are aY, daughter is either Aa but could equally be AA. This is a 50% chance for her. Carrying that forward her unaffected partner is AY. There is no chance the girls will be affected, but there's a 50% chance that they will also be carriers. There is a 50% chance the sons will be affected if the mother is Aa. If the mother is AA then the recessive trait is lost in that line. Therefore, 50% x 50% gives 25% for sons and 0% chance for daughters.
If the sons choose to have children, depending on their partner trait status, their girls will be carriers or affected, and the sons will be unaffected (if no recessive gene carried) or affected 50% if heterozygous or 100% affected if homozygous recessive.