Final answer:
The risk for hemophilia A in the grandsons whose paternal grandfather is hemophiliac, is determined by if their mother is a carrier. A carrier mother has a 50% chance of passing the defective gene to her son, so the risk in the grandsons is 1 in 4 or 25%.
Step-by-step explanation:
When assessing the risk for hemophilia A in grandsons whose paternal grandfather is a hemophiliac, we must understand that hemophilia A is an X-linked recessive disorder. Since the grandfather has hemophilia, he has the abnormal X with the hemophilia allele. His sons, who are the fathers of the grandsons in question, would not have hemophilia since they received their father's Y chromosome. However, their daughters (the grandsons' aunts) could be carriers if they inherited the X chromosome with the hemophilia allele.
For a grandson to have hemophilia A, his mother must be a carrier of the disease. A carrier mother has a 50% chance of passing the X-linked recessive hemophilia allele to her sons. So, if the grandsons' mother is a carrier, each male child has a 1 in 4 (25%) chance of having hemophilia because there's a 50% chance that the mother is a carrier (daughter of a hemophiliac male) and another 50% chance that the son inherits the affected X chromosome from a carrier mother.