Breeding of slaves has nothing to do with the preponderance of black athletes in the NFL and NBA.
It's because of culture and because perception tends to become reality overtime. Attributing that to slavery or "breeding" is fallacious. From the 1910s-1940s Jews dominated boxing (in fact there were 26 Jewish world champs), I doubt anybody would say it's because of their genes or because Jewish people are unusually athletic. Another example - there are few black pro swimmers. In fact, around 60% of black kids can't swim, compared to only about 31% of white kids. It would be easy to postulate that this is because of some physiological advantage that white people have, but from a scientific standpoint it would be bullshyt. More do to with black kids being less likely to have regular access to a swimming pool, and less interest in swimming in general due to a cultural preference for other sports.
Whenever a certain ethnic group dominates a certain sport, it's going to be mostly cultural. Not trying to diss, but it's kind of a pet peeve of mine when people automatically equate things to genetics that have little to do with it.