Artificial selection in dogs

I like this higher-order question, but that's the beauty of artificial selection, isn't it? That is, how we can select the traits we want in such ways that will ultimately benefit us at the end. We could technically breed two very smart breeds and produce a relatively smart offspring, but this could backfire on us in several ways. What if the new breed is aggressive in nature, or isn't physiologically capable of controlling his bladder or sphincter (owners may not want this). The intelligence genes may also interfere with other traits, like their ability to reproduce.

First I'm not necessarily saying they have to be two different breeds they could just be the brightest offspring of a single breed.  The secondly it's not like the fear of producing a bad breed has ever stopped us before there's already uncountable numbers of domesticated animals species.  If something does go bad then we stop breeding the dogs and try species no harm done.