Science & History
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Deconstructing the Gene Pool
Dr. Mark Neff and his team uncover the surprising origin of a potentially deadly mutation


Until the mid-19th century, any hodge-podge of similar-looking dogs performing similar tasks was awarded the right to be called a breed. However, as inventions (such as guns) mechanized jobs that dogs normally performed, many breeds—like the tumbler, who “tumbled and turned” to mesmerize prey—simply sank back into the ancestral soup, taking their unique traits with them.

One of these ancient breeds, the glacier-climbing Lundehund with its unusual polydactyl triple-jointed toes, survived, but its current population is so small that the breed teeters on extinction’s edge. And a few, like the ubiquitous working collies and spaniels of Great Britain, spawned a number of the breeds created during the prosperous, class-conscious Victorian era. In the age of upward mobility, those on the way up claimed many of the privileges of the upper class, including the luxury of breeding and showing dogs.

More than one-quarter of the world’s estimated 375 breeds were created between 1859, when the first dog show was held in the UK, and 1900, when Westminster and Crufts were well established; even the most subtle differences in weight or color were enough to allow registry of a new breed type. In many cases, the subdivision of farm dogs was an unintended consequence of competitive exhibition in dog shows.

Responding to the shows’ strict criteria for body type, size and color, breeders drew from an increasingly smaller number of founder populations to create dogs who conformed to these standards. Breeding closely related dogs to one another became a popular way to refine a breed, which today means a group of dogs with a common gene pool and characteristic appearance and function.

Unfortunately, the down-side of homozygosity (having two identical genes at a specific location on the DNA strand) is disease and unsoundness. As a consequence of this intense concentration on form, modern dogs suffer from more than 350 genetic illnesses, and today’s breeders bear the burden of restoring their lines to health.

There are no easy answers. Removing affected individuals from breeding populations may decrease the incidence of a particular disease, but smaller gene pools create opportunities for other congenital problems. In cases where an entire breed is afflicted, out-crossing with other breeds means running the risk of losing truly unique traits, such as the Lundehund’s joint flexibility.

Recent research has shown that a single mutated gene, unnoticed for over a century, is responsible for sensitivity to several modern medicines, ranging from ivermectin (a common ingredient in heartworm preventatives) to anticancer agents such as vincristine. These adverse drug responses can cause illness or death in the breeds that harbor the mutation.

A team of researchers led by Professor Mark Neff at UC Davis expanded the results of earlier research by demonstrating that the mutation probably originated in a single generic herding dog who lived in Great Britain in the mid-1800s. This dog must have been a common ancestor of founding dogs for nine different breeds, all of which were found to possess the mutation. Moreover, scientists involved in this study were able to describe the frequency of the mutation in these various breeds, further defining the inherited risk of adverse drug response: Collie (54.6%), Long-haired Whippet (41.6%), Miniature Australian Shepherd (25.9%), Silken Windhound (17.9%), McNab (17.1%), Australian Shepherd (16.6%), Shetland Sheepdog (8.4%), English Shepherd (7.1%) and the Old English Sheepdog (3.6%).

Dr. Neff talks about his research and the implications of genetic testing on the health and well-being of dogs.

Jane Brackman: In addition to helping breeders make informed decisions, your findings provide an opportunity for veterinarians to treat dogs based on their individual genetic profile. What do you mean when you say dogs can be treated with personalized medicines?




Jane Brackman, PhD, is an authority on the cultural history of canine domestication and the author of two books on pets in 19th-century America. See her new pup, Barkley, and watch him grow on her blog.


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