Assume that a mutation occurs in a human population that produces a horn (like a unicorn) in the middle of the forehead. further assume that the horn is inherited as a y-linked trait in humans. a man with the horn has a daughter who has a son with a man who has lacks the horn. what is the probability that the horned man's grandson also has a horn?

0% that the offspring will have a horn. Since the mutation is y-linked, that means that the carrier of the mutation has to have a y chromosome. Since the man's child is a daughter, that means that she didn't get the mutated y chromosome. So none of her children will get the mutation from her father. And since the father of the grandchild doesn't have the mutation, that means that the child can not have the mutation. Hence, there's a 0% chance of the grandchild having the mutation.