Where Do Redhead Babies Come From?

[FRDGAL]

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'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."