Mail-man syndrome

When a child in the family looks nothing like the father or siblings.
It implies that the mail-man is really the father of that child.

the mother is blonde and green eyed, the father is brunette and brown eyed, child 1 is brunette and green eyed, child 2 is blonde and brown eyed; however, little Bobby is a ginger.

People would say he has "mailman syndrome" or call the situation in general "mail-man syndrome"

It would be especially appropriate if the mail man was also a ginger.

Mail-man syndrome by Your Mail Man January 18, 2010

Mailman syndrome 

|ˈmālˌ*manˈsin*drōm|
noun

The condition in which the man a person believes to be their father is not their biological parent.

“John is 6’4, blonde with blue eyes while his dad is 5’5, black hair, and brown eyes; total Mailman syndrome.”

Mailman syndrome by Osiris Black August 24, 2009