A missionary gets sent into deepest darkest Africa and goes to live with a tribe there. He spends years with the people, teaching them to read, write, and the good Christian ways of the white man. One thing he particularly stresses is the evils of sexual sin. Thou must not fornicate or commit adultery! 

One day, the wife of one of the Tribe's noblemen gives birth to a white child. The village is shocked and the chief is sent by his people to talk with the missionary. 'You have taught us of the evils of sexual sin, yet here a black woman gives birth to a white child. You are the only white man that has ever set foot in our village. It doesn't take a genius to figure out what has been going on!' 

The missionary replies: 'No, no, my good man. You are mistaken. What you have here is a natural occurrence - what is called an albino. Look over there to your fields. You see a field of white sheep, and yet amongst them is one black one. Nature does this on occasion.' 

The chief pauses for a moment then says, 'Tell you what, you don't say anything about the black sheep, and I won't say anything about the white child.'