Where do red-headed babies come from?

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!!