Though it tends to be more distant than the European ancestry found in many African Americans. The majority of European ancestry found in African Americans can be traced to the Civil War era, before Jim Crow shoveled all mixed race people into one category or another based on overt racial characteristics. Genetic testing has become refined enough to the extent that people can trace their ancestry to discreet areas around the world. Black Americans, for example, can find the specific tribes in Africa where their ancestors lived. Historically it's been far more difficult for African Americans to do this, because census records were never kept for slaves.
Incidentally, a lot of this work has been championed by Gates himself, who's main academic work is uncovering uncovering African American history. You can find a number of clips on YouTube from his "African American Lives" series that was aired on PBS a few years ago.
Here's one with Chris Rock:
http://www.youtube.com/watch?v=YmCPDe3_-qo