It has already been done in the USA. At least for physical attributes. I do not know if it is possible to increase mentality the same way, It soon will not be necessary, for science will be able to correct genetic problems with specific people. (correct the genes so a person is not going to have heart problems even though both parents had heart problems) For the physical see this website
https://www.quora.com/Were-Black-slaves-bred-for-athletic-performance