A missionary gets sent into deepest, darkest Africa. He spends years with the people, teaching them to read and write, and about the good Christian ways of the white man. One thing he particularly stresses is the evils of sexual sin, "Thou must not commit adultery or fornication!"
One day the wife of the Village Chief gives birth to a white child.
The village is shocked and the chief is sent by his people to talk to the missionary.
"You taught us of the evils of sexual sin, yet here is a black women who 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 work out what has been going on!"
The missionary replies: "No, no, my good man. You are mistaken. What you have here is a rare, but natural occurrence. It's called an albino. Look to thy yonder field. See there a field of white sheep, and yet amongst them is one black sheep. Nature does this on occasion."
The chief pauses for a moment and then says, "Tell you what, my friend, you say nothing about the black sheep, and I'll say nothing about the white child!"