Ann Coulter debunks a popular myth. Read the whole thing, but here's the bottom line.
In response to DNA proof that only one of Hemings’ children was related to any Jefferson male -- and her firstborn son was definitely NOT fathered by any Jefferson -- the Thomas Jefferson Foundation, the Monticello Association and the National Genealogical Society promptly announced their official positions: Thomas Jefferson fathered all six of Hemings’ children! Guided tours of Monticello today include the provably false information that Jefferson fathered all of Hemings’ children.
So now you, at least, know the truth -- not that it matters in the slightest. Happy Fourth of July!