I know little about genetics, except for the two overriding
principles--hybrid vigor and "regression to the norm". That is, if you "breed
the best to the best and hope for the best", as espoused in racehorse
breeding, you don't get the best, you get a regression to the norm of the
families represented. The only hope for new vigor in a breed, as I understand
it, is outcrossing.
Of course, when you suddenly get the best, through outcrossing, you then line
breed it for a while, firming up the improvements.
ti