Did you look at the BBC link I posted? I watched the documentary that went with this film a couple of years ago and it was eye-opening. Dogs are bred that have brains too big for their skulls, others cannot deliver without assistance, still others suffer breathing issue all in the pursuit of a certain look.
Mixed breeds are by no means immune to genetic diseases ( I never made that claim) but they do have a greater genetic pool to draw from so are less likely to suffer from issues related to inbreeding that are actually common in pedigreed animals.
Don't take my word for it - do some googling. Here's one vet's opinion. I've been told the same thing by my own vet. YMMV
"To the annoyance of most purebred breeders, most of us tend to side with the mixes (and, no, we don't consider hybrids, like Labradoodles, exempt from the purebred designation). Mutts mostly win out for their sheer hardiness, thriftiness and longevity, due to their lower incidence of genetic disease.
Since they are likely to have a lower risk for receiving a high dose of specific purebred genetic material that can lead to inherited disease, I believe they’re healthier than purebreds in this particular sense. As a result, many mutts have a lower rate of hip dysplasia, certain knee diseases, most spinal diseases, many heart diseases, plenty of cancers and a whole lot of skin, blood, brain, liver and kidney diseases, among others.
"Statistically speaking, mixes win out — but genetics are only half the battle. Here are some other key factors that come into play in the mutt vs. purebred discussion."
Source:
http://www.vetstreet.com/our-pet-experts/the-great-debate-are-mutts-healthier-than-purebreds