Actually I think I should be talking alleles instead of genes......but that was another chapter....
The theory your prof is talking about is hybrid vigor. But... it does not take in to account a breeder that makes wise decisions and there are other things to consider such as: ...."
However, some deleterious recessives are common across many seemingly unrelated breeds, and therefore merely mixing breeds is no guarantee of genetic health. The declining overall health of many purebreds is also leading to a decline in the mixed-breed population, especially with the "designer dog" fads. Breeding two poor specimens together does not guarantee the resulting offspring will be healthier than the parents and in fact, due to simple genetics law, the offspring actually stand to inherent the worst of both parents. This is most commonly seen in "pet store puppies". Not all damaging genes are recessive, and there are relatively few single-gene traits. Purebred and mixed-breed dogs are equally susceptible to non-genetic ailments, such as rabies, distemper, injury, and infestation by parasites." from wikipedia..... but there are some genetic predispositions that are going to show up more when closely bred. Have to admit it.