Final answer:
Barbara's risk of being a carrier for Hemophilia A is 50%. Since her father, Alan, has the condition and her mother may be a carrier, Barbara has a 50% chance of having inherited the carrier X chromosome from her mother.
Step-by-step explanation:
Hemophilia A is a X-linked recessive disorder, meaning that females need to inherit the defective gene from both parents to have the condition, while males with one defective gene will express the disease. In the scenario provided, since Alan has hemophilia and is the brother of Alice, we can deduce that their mother was a carrier of the hemophilia gene. Assuming Alice inherited this gene, there is a 50% chance that each of her children could be carriers. As Alice's son, Blaine, has hemophilia, Alice must be a carrier. Therefore, Barbara, Alan's daughter, has a 50% chance of inheriting the defective gene from her mother, Annette.
Since Barbara's mother Annette had children with Alan, who has hemophilia, and their daughter (Barbara) does not, Barbara must have inherited a normal gene from her father (as he can only contribute a Y chromosome or his affected X chromosome, the latter would have resulted in Barbara having Hemophilia A). However, Barbara could have still inherited the carrier X chromosome from her mother. Therefore, there is a possibility that Annette, the mother, is a carrier, possibly providing Barbara with a carrier X chromosome. As such, Barbara's risk of being a carrier is 50%.