@lvoss:
Hybrid Vigor is often used incorrectly as a selling point for mixed breed dogs. In order to have Hybrid Vigor the two parents would have to of different species. In other words, mules, ligers, tions could exhibit "hybrid vigor" because they are truly hybrids the mixing of two different species. Dogs can not because they are not different species.
As for the idea that mixed breeds will be healthier than mutts. That is only true if all inherited disorders are simple recessives and the disorders exist only in one breed. Since all dogs are of the same species this is not a good assumption. Some disorders are dominant, some are polygenic and most are in many breeds with a difference in the frequency of the gene so though a disorder may be rare in some breeds the gene can still be there. So it may be that some mixed breeds are healthier than their parents but some are actually less healthy than either of their parents.
THANK you Lisa! I was so dreading having to find the right words to explain what you have so clearly stated above!
You can't have a hybrid with parents of the same species!
I can think of several inherited diseases that happen in occur in mixed breed dogs as well as pure bred dogs: thyroid disease, autoimmune diseases, hip dysplasia, and of course cancer (though I suppose it is debatable about it's level of inheritance)