I just want to say that this is a popular myth and isn't true at all LOL. When it comes down to it, it all depends on the pedigree and breeding and genetics, nothing to do with "pure bred" or "mixed". You can have a mixed dog with genetically poor hips, eyes, elbows, or other health problems, you can have a pure bred dog from a pedigree of health tested, physically sound dogs.
The funny thing is that for people who know a thing or two about dogs, this is actually one of the reasons why some people may choose to find a reputable breeder for a puppy with genetically healthy parents/dogs in their pedigree. With mixes or adopted shelter dogs, it's a crapshoot- you may get lucky, you may not, because its genetics are unknown. And of course, there are others who breed dogs even while knowing they may be passing down major physical flaws or health issues, and that is a bad breeder.
Anyway, no idea at all if it applies to other animals in such a specific and vast way like it does for dogs, but that's how it is with dogs haha thought I'd clear it up in the thread!