Sigh.

Joseph Ellis' book was researched and written BEFORE the DNA testing was done. Ellis is also recognized as one of the traditional primary deniers of the Jefferson/Hemings liason. See Annette Gordon-Reed for additional information on Ellis' biases and mis-interpretations.

Also:

That early, firsthand experience with the interplay of race and history informs much of Gordon-Reed's work, including her compulsively readable new book, "The Hemingses of Monticello(2008)," in which she traces the family history of Sally Hemings, the slave who had a 38-year relationship with Thomas Jefferson. Gordon-Reed, a former lawyer, also edited an anthology of essays about race and the law, and co-wrote Vernon Jordan's 2001 memoir, "Vernon Can Read!" But she is best known for 1997's groundbreaking "Thomas Jefferson and Sally Hemings: An American Controversy," which examined historians' treatment of the Jefferson-Hemings liaison, and made a strong case that Jefferson fathered seven children with Hemings. DNA testing a year after the book came out vindicated Gordon-Reed's assertion, and made her book a cause célèbre among Jefferson scholars. Joseph Ellis, whose National Book Award-winning biography of Jefferson, "American Sphinx(1998)," claimed Jefferson never slept with Hemings, later conceded the point, writing that it was difficult not to conclude Jefferson had been "living a lie."

From- http://www.newsweek.com/2008/10/03/a-lawyer-s-new-jefferson-memorial.html

See also :http://www.friesian.com/ellis.htm for aditional information on Ellis' biases. A simple (Simple?) web search will turn up lots more in this vein.