An interesting thing about History is the way in which legend and speculation become "fact". A perfect example is the Sally Hemings - Jefferson controversy. I just saw C-Span show in which two Jefferson scholars simply stated as undisputed fact that "of course, Jefferson had an affair with Sally Hemings".
The problem is that it isn't a proven fact. Nor did the DNA testing "prove" it. Here is a link to rebuttal to the Original research report on Jefferson-Hemmings DNA study. In summary, the DNA established the following:
1) Someone with Jefferson DNA was the child of several Hemings children
2) One of Hemings children (who claimed to be Jefferson's son) had no Jefferson blood.
That's it. Possible alternative explanations for several of Hemings children having Jefferson family DNA:
a. Hemings was a quadroon who had Jefferson DNA to start with; or
b. another Jefferson relative was the father
Other than an oral tradition of several Hemings children, no other evidence exists that Jefferson was the father of some of her children. Further, Jefferson and his family denied it.
And there is no pattern of Jefferson before Sally Hemings of having affairs with Slaves. So why the certainty? Basically, Politics and to sell books.