In some cases, these health problems arose as byproducts of inbreeding. Because breeds are genetically closed populations, a disease-causing mutation that just happens to pop up in one dog can quickly become common in future generations. “Especially if the animal with that mutation is otherwise a prize specimen,” Dr. Serpell said. “Because everyone will want to breed from that individual.”
The result is that many modern breeds suffer from ailments that dog lovers never would have deliberately selected for. Labrador retrievers are prone to a degenerative eye disease known as progressive retinal atrophy, while Cavalier King Charles spaniels often develop a heart condition called mitral valve disease.
In other cases, however, the physical traits that breeders are seeking can themselves be the problem, especially as these characteristics have become increasingly exaggerated. The breed standard for bulldogs explicitly calls for “very short” muzzles, but, over time, the dogs’ snouts have all but disappeared, Dr. Serpell said, resulting in serious respiratory problems.
Mutts and mixed-breed dogs can be healthier than purebred ones, especially if they have small amounts of DNA from many different types of breeds, Dr. Bannasch said.
But some mixed-breed dogs may still be highly inbred, she said, and some health problems are common in multiple breeds. Many large breeds are prone to bone cancer, for instance; mixing a few of them together may not have much benefit. “You can’t lump all ‘mixed breeds’ together,” she said.
The link between breed and behavior is murkier.
In creating modern breeds, humans generally put a much stronger emphasis on appearance than behavior, which is also shaped by a dog’s training and early environment.
“Behavioral traits definitely vary from breed to breed, but not nearly as strongly as the morphological traits do,” Dr. Boyko said. “You’re never going to get a collie that looks like a Great Dane,” he added. “But I see lots of dogs exhibit pointing behavior that aren’t pointers.”
Scientists have uncovered some general patterns. Terriers, which were bred to hunt and kill pests, are more likely to exhibit “predatory chasing” than herding dogs, for example. And, on average, Siberian huskies are more likely to howl than Labrador retrievers are.
But, overall, breed is a poor predictor of behavior, and there is more variation within breeds than between them, scientists have found. Someone who wants a friendly, fetch-loving dog that doesn’t bark can’t bank on getting exactly that just by bringing home a Labrador retriever. “You may get exactly the opposite of that,” Dr. Lord said.