Out of curiosity why are blacks allowed to say the "N" word but whites are not?

What about if you have a black Dad and a white Mom, is the son or daughter allowed to say it?

In November 2006 Jesse Jackson wanted to ban the "N" word. Yet, according to reports he used the N-word during a break in a TV interview where he criticized presidential candidate Barack Obama, Fox News confirmed Wednesday.

So I ask if Jesse Jackson is going back on his word then it must be ok to say that word, but how is it ok for?