The oldest three are tall with red hair and light skin while the youngest son is short with black hair and dark eyes.
The father was on his deathbed when he turned to his wife and said,
"Honey, before I die, be totally honest with me: Is our youngest son my child?"
The wife replied, "I swear on everything that's holy that he is your son."
With that, the husband passed away. The wife muttered, "Thank God he didn't ask about the other three."