Actually there exists no notion of a human of "pure race". A dog breeder told me that a thoroughbred poodle is not characterized by all ancestors being poodles - how could they? - but by certain genes being inherited from both parents alike, as defined by the breeder who created the "race".

The more interesting question for presidents is whether they have a consciousness of slaves in their close ancestry, because that will inevitably "colour" their view of the world. Obama does not seem to qualify here, although some kind of "slave", in 17th century, was alleged in his mother's line on another Mudcat thread.