What I mean by this is that race is a social construct. In the United States, there's the "one drop" rule which means that if any of your ancestors are considered non-white (by the ever-changing standard of the day) then you are not considered white. (If you doubt that this rule is still widely used, start checking school records.) For example, Dan O'Brien had one "white" and one "black" parent, but is considered "black". Implicit in this is that whiteness is something that can be tarnished. By standard racial catgorization, no Japanese runner is "white", and a few generations ago a Canadian of Greek heritage would not have qualified either.
To stand the notion on its ear, I'd bet that none of the American sprinters who have broken 10.00 have 100% African ancestry. Furthermore, if the African genetics are so important, why are American, British and Caribbean sprinters so much better than African ones?