I'm really confused about this with regard to mixed breed dogs (a/k/a "designer dogs"). It does seem somewhat logical to me that if you breed two different breeds together that don't have the same hereditary health issues, you'll end up with pups that don't have either of the breed's health issues, demonstrating hybrid vigor. BUT, if you breed two breeds that DO have the same hereditary health issues, you're going to end up with pups that could be affected/afflicted by the issue.
I started thinking about this with regard to Fanconi Syndrome actually. I had always thought that hybrid vigor was a "selling point," so to speak, and completely untrue, but from what I've learned (or, at least, thought I learned, about the way FS can be passed on), breeding (doG forbid!) an FS-affected Basenji dog with, say, a Shiba Inu bitch (which, as far as I know, is a breed not known for FS), would get you puppies that can't possibly have FS and, thereby, exhibit hybrid vigor, by being more healthy than their sire.
I realize the above is a very simplistic view of how genetics work and that there's much more involved than I know. Can anyone enlighten me? Thanks!