I think you may be thinking of something called Hybrid vigor, but this works only if natural selection is in operation.
"Don't be duped into thinking that mixing breeds will result in healthier offspring. The truth is that puppies, whether purebred or mixed, inherit genes from each parent. If the parents pass along the genes for a hereditary disease, the pups stand a good chance of having that disease.
Hybrid vigor—the idea that unrelated parents produce healthier offspring—works only if natural selection is in operation. In the wild, a dog with clinical symptoms of a debilitating disease such as hip dysphasia or epilepsy would not be able to survive, and so would not pass on its genes. But in the modern world, with reasonable veterinary care and a constant food source, dogs with serious, even deadly, problems do survive long enough to produce lots of pups. "
Purebred vs. Mixed Breed and other pet resources - FamilyEducation.com