It’s Rust

“Doctor,” the man said, “I don’t mind telling you, but I’m a little upset because my daughter has red hair. She can’t possibly be mine.”

“Nonsense,” the doctor said. “Even though you and your wife both have black hair, one of your ancestors may have contributed red hair to the gene pool.”

“It isn’t possible,” the man insisted. “This can’t be, our families on both sides had jet-black hair for generations.”

“Well,” said the doctor, “let me ask you this. How often do you have sex?”

The man seemed a bit ashamed. “I’ve been working very hard for the past year. We only made love once or twice every few months.”

“Well, there you have it!” the doctor said confidently. “It’s rust.”