Nature, nurture, can be a compilation of either or both.
There are FAR my carcinogenics being allowed in food today. Human and with an even more lenient allowable ingredients containing these in dog food.
To blatantly paint a broad brush across 'responsible' breeders is unconscionable.
That said.
For those breeders that DO NOT breed for money they have independently spent millions of dollars and time finding and instituting genetic testing within their specific breed AND TEST FOR IT and eliminate any animal that posses an unwanted hereditary health issue gene.
Again that is a very select group of breeders. Buying a pure breed dog out of the local newspaper or on line is NOT any of them.
And AKC registered dog from this select group that tests will actually have the letters of what the dog was tested for, cleared of, right after the dogs name.
Over the past 20 years genetic testing for specific breeds 'paid for by these select breeders and their clubs' has produced results of MANY of these markers to make their way over successfully to human testing. Your welcome.