Daniel should be buying a present for John, as John is his father-in-law. Amy, being Daniel's wife, is John's daughter, which makes John Daniel's father-in-law. Ethan should be buying presents for two people: his father, Daniel, because that is the direct father-son relationship, and his grandfather, John, because John is his mother Amy's father, making him Ethan's grandfather on his mother's side. Mark is not mentioned as having any children, so there are no gifts to be bought for him on Father's Day.
In a family, John has two children, Amy and Mark. Amy is married to Daniel, and they have a son named Ethan. Mark is single. On Father's Day, Amy and Mark decide to buy presents for their father, John. Considering the relationships, whose father should Daniel and Ethan be buying presents for on Father's Day?
Respond very briefly.