That's why anyone who says Thomas Jefferson slept with Sally Hemings is speaking on the basis of something other than science.
I know some of the "extended Jefferson family" quite well. The belief is akin to a religious one, a prideful attitude and one that developed virtually exclusively within the black community of a couple hundred or so years back. As is the case with many issues, science will provide the facts, but people will provide the beliefs. The challenge from science is about as welcome to these putative descendants as would be Drawin at a creationsts assembly.