In some cases, these health problems have arisen as a by-product of inbreeding. Because the breed is a genetically closed population, the disease-causing mutations that happen to appear in one dog may soon become common in future generations. “Especially when the animal with that mutation is otherwise a specimen of the award,” Dr. Serpel said. “Because everyone wants to breed from that individual.”
As a result, many modern breeds suffer from illnesses that dog lovers have never intentionally chosen. Labrador retrievers are prone to degenerative eye disease known as progressive retinal atrophy, and Cavalier King Charles Spaniels often develop heart disease, known as mitral valve disease.
However, in other cases, the physical properties that breeders are seeking can be problematic, especially as these properties are increasingly exaggerated. The bulldog breed standard explicitly calls for a “very short” muzzle, but over time the dog's nose almost disappeared, causing Dr. Serpel to cause serious respiratory problems.
MUTT and mixed breeding dogs can be healthier than purebred dogs, especially if they have small amounts of DNA from many breeds, Dr. Bannash said.
However, some mixed dogs may still be very inbred, she says, and some health issues are common in multiple breeds. For example, many large varieties tend to have bone cancer. Mixing some of them together may not have much advantage. “We can't put all 'mixed varieties together,” she said.
The link between variety and behavior is muddy.
In creating modern breeds, humans generally place a much stronger emphasis on appearance than behaviours shaped by dog training and early environments.
“Behavioral traits definitely vary from one variety to another, but not as strong as morphological traits,” Dr. Boyko said. “You're never going to get a collie that looks like a great Dane,” he added. “But I see a lot of dogs showing pointing behaviors that are not pointers.”
Scientists have discovered several common patterns. Terriers raised to hunt and kill pests are more likely to exhibit “predatory pursuits” than dogs, for example. And, on average, Siberian husky is more likely to do than Labrador retrievers.
Overall, however, varieties have poor predictors of behavior, with more variation among breeds than scientists have discovered. Those who want a dog who just takes home a Labrador retriever and likes friendly fetching that can't be barked, can't put their bank in the bank by getting exactly that. “You might be the exact opposite,” Dr. Lord said.
