I think it is both. I think that you can have a well-bred pup with issues. And you can have puppies out of a litter have issues due to genetics. I think that a good breeder does their best to reduce the incidents of health and temperament issues.
I also think that two people could take the same puppy and have two very different results.
I think that the more puppies you produce, the more puppies you have out there, the more chances of having some with health or behavioral problems.
On the other hand, if you produce very few puppies, you can't possbily be making giant strides in reducing health concerns, as the dogs you are breeding, and the lines that you are breeding, well, you would be done before any knowledge becomes useful.
Cancer, well it may have a basis in genetics, but it may also be seriously impacted by the procedures, foods, vaccines, preventatives that we are stuffing into our dogs. Most of us have at one point bought dog toys made in China, where they are probably painting them with lead based paint. We use products on are carpets, furniture, and lawns that may be causing issues. It is hard to pin point the genetics when there are so many things that may be in common when it comes to how pups are raised.
I guess the answer is to look up what to look for in a breeder, and put your breeder to the test. If he is decent in most of the areas, or all of the areas, and you are happy with your dog, go with him. If he has serious deficiencies, or you are not pleased with the puppy you got from him, then go with someone else. Probably if you are asking this question, you should maybe try another breeder, with the understanding that this is a live creature and there really are no guarantees.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.