[pmwhite]

It is a little handwavy, but perhaps the BG worked for generations to achieve key areas of hybrid superiority by honing pure breeding lines in these two families? Does not seem to be what the BG are doing, though.

Furthermore, one can now ask if the more perfect specimen for a given environment might be a chimera: this genome for nervous system, that genome for leg muscles, this other genome for internal organs, etc.

But your main point stands. The Dune series presumes that such a thing as breeding a superior human is not very difficult to manage on the scale of mere centuries. I am not sure how even define such a thing. And I rather doubt such a specimen would breed true with a similarly superior specimen.