Question Detail

Hypertrichosis pinnae is a Y-linked trait. Joseph passes his Y chromosome to his sons. His sons will then pass the Y chromosome to their sons (Joseph's grandsons). So, if Joseph has a son, that son will inherit the trait. If that son has a son, the grandson will inherit the trait. So the chance depends on the chain. But the question asks the chance that his grandson will inherit the trait from him. If Joseph has a son (who gets the Y), and that son has a son, that grandson will have the trait. So it's not 0%. If we assume Joseph has at least one son, and that son has a son, then the chance is 100%. But without assumptions, if we consider a random sperm, Joseph's son gets the Y 100% of the time. That son's sperm has a 50% chance of giving a Y to a grandson. So overall, Joseph's chance of a grandson with the trait depends on the children's sexes. The question is poorly worded, but Y-linked traits are passed from father to all sons. So a grandson can only get it if Joseph's son (who has it) has a son. So the chance is not 100% overall. Typically, for a Y-linked trait, if Joseph has a son, that son has the trait, and any son of that son will have the trait. So the probability that a specific grandson inherits it is 1 (if Joseph's child is a son) * 1/2 (if that son has a son) = 1/2. So 50% is possible. But the question might be trick: A grandson inherits his Y from his father (Joseph's son). So the chance is 100% if Joseph has a son who has a son. Without knowing family, it's ambiguous. But often in such questions, they mean the probability that Joseph's Y chromosome reaches his grandson, which is 1/2 if Joseph has a son and that son has a son. But the options include 0%, 25%, 50%, 100%. 50% is a common answer for this kind of question. However, note that if Joseph has a daughter, the line stops. So for a random grandchild, the chance is lower. But if we assume Joseph has a son, and that son has a child, the chance that child is a son is 50%. So answer might be 50%.