A Black Sheep

A professor is sent to darkest Africa to live with a primitive tribe. He spends years with them, teaching them reading, writing, math and science.
One day the wife of the tribe's chief gives birth to a white child. The tribe is shocked, and the chief pulls the professor aside and says, "Look here! You're the only white man we've ever seen and this woman gives birth to a white child. It doesn't take a genius to figure out what happened!"
The professor replied, "No, Chief. You're mistaken. What you have here is a natural occurrence, what we in the civilized world call an albino! Look at that field over there. All of the sheep are white except for one black one. Nature does this on occasion."
The chief was silent for a moment, then said, "Tell you what. You don't say anything more about that sheep and I won't say anything more about that white child."