Most Dog Breeds Are Super Inbred, Study Finds

English bulldogs are especially prone to breathing and other health problems due to their unique…

English bulldogs are especially prone to breathing and other health problems due to their unique body and facial shape created through breeding.

English bulldogs are especially prone to breathing and other health problems due to their unique body and facial shape created through breeding.
Photo: Jacob King/PA Wire (AP)

New research seems to confirm the idea that many of our cutest purebred pups are also burdened by their genetics. The study determined that most dog breeds have high levels of inbreeding. What’s more, this inbreeding can contribute to various health problems and expensive vet bills over time, particularly for larger dogs.

It’s certainly no surprise that some dog breeds aren’t blessed with robust health—often the result of long-running breeding programs that use closely related relatives to select for the traits most liked in a breed. Brachycephalic dogs like the pug or bulldog are well known for their increasingly smushed-in faces, for instance, a feature that predisposes them to breathing problems (not to mention their other issues). This new study, a collaboration between veterinary researchers in California and Finland, decided to take a broader look, hoping to get a sense of how much of an impact inbreeding is having on the purebred dog population in general.

To do this, they turned to a genetic database made up of results from commercial DNA tests of nearly 50,000 dogs, encompassing 227 breeds in total. Then they analyzed the average genetic similarity of dogs within a breed in order to estimate their level of inbreeding on a percentage scale from 1 to 100. To further check their math, they compared their results to data from past studies that studied smaller groups of breeds.

Overall, they estimated that the average level of inbreeding within these breeds was around 25%, or about the amount of genetic similarity you would see between two siblings. But while it’s fine for two members of a family to be that close, it doesn’t bode well for a population of animals that rely on genetic diversity, as most do. At levels far lower than that for humans (around 3% to 6%, the authors say), you can start to see a higher risk of hereditary disorders or other conditions influenced by genes, like cancer.

“Data from other species, combined with strong breed predispositions to complex diseases like cancer and autoimmune diseases, highlight the relevance of high inbreeding in dogs to their health,” said study author Danika Bannasch, a veterinary geneticist at the University of California, Davis, in a statement from the university.

Bannasch and her team went even further in their research, by cross-referencing their findings with data from a pet insurance database, using insurance claims for non-routine vet visits as a proxy for dog health. Dog breeds with higher levels of inbreeding were more likely to need added vet care than others, they found, though factors like size played a key role, too. Brachycephalic breeds were also less healthy than non-brachycephalic dogs on average.

“While previous studies have shown that small dogs live longer than large dogs, no one had previously reported on morbidity, or the presence of disease. This study revealed that if dogs are of smaller size and not inbred, they are much healthier than larger dogs with high inbreeding,” said Bannasch.

The team’s findings were published in Canine Medicine and Genetics.

Bannasch and her team did find some dog breeds that were much less inbred, such as Danish-Swedish farmdogs. These dogs are likely better protected due to a large establishing population and because they’re still bred for various jobs, not simply for their appearance. And not all breeds with high inbreeding seemed to be more unhealthy as a result.

But the authors say “careful management of breeding populations” is needed to preserve the existing genetic diversity of all these dogs, both in educating breeders and in the use of genetic screening to keep an eye on inbreeding levels. Some breeders have started to outcross their dogs (breeding with dogs outside the pedigree) in hopes of improving the genetic health of purebred populations, but the authors caution that even these efforts have to be carefully watched to ensure they will really improve diversity.

Though there has been some recognition of the problem, dog breeding groups and organizations have been hesitant to admit many flaws with the current state of things. In response, some veterinary groups have even started to plead with people not to buy popular breeds like the pug, while at least one country, the Netherlands, has passed strict laws on breeding brachycephalic dogs.