I apologize I brought up the dog thing haha
But this is a myth- mutts are just a random draw, like buying raffle tickets. It totally depends on the individual unknown genetics of the mutt, just like it depends on the individual genetics of the purebred. Many purebreds that come from long lines of health tested and proven dogs are setting their chances high of producing more healthy dogs. While mutts are never bred ethically/are randomly bred/some stray dogs had puppies on the street, so really are luck of the draw and mostly have lower chances of being healthy than well-bred purebreds.
There are well bred and poorly bred purebreds.