Slaves being the most obvious group, but I guess I don't want to limit my question. I knew slaves were sold at higher or lower prices according to physique and ability, but were there attempts to "breed out" unwanted traits like, say, aggressiveness?
The word you are looking for is "eugenics." That is not a good answer, but hopefully knowing the terminology will help you find answers more easily if you search /r/askhistorians.