A Father's Last Request

A husband and wife had four boys. The odd part of it was that the older three had red hair, light skin, and were tall, while the youngest son had black hair, dark eyes, and was short.
The father eventually took ill and was lying 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 then muttered, "Thank God he didn't ask about the other three."