Rust?

After their baby was born, the panicked father went to see the obstetrician.
"Doctor" the man said, "I don't mind telling you, but I am 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, this can't be". the man insisted.

"Our families on both sides have had jet black hair for generations".

"Well how often do you have sex with your wife", asked The doctor.

The man seemed a bit embarrassed.
"I have been working very hard for the past year. We only Make love once or twice every few months".

"Well there you have it", the doctor said confidently, "Its Rust".