THE genetic signatures that produce breeds of dog as diverse as the chihuahua and rottweiler have been mapped by scientists in a study with important implications for canine and human health.

A detailed comparison of the genomes of 85 breeds has uncovered their unique DNA profiles, which explain the vast physical and behavioural differences between mastiffs and labradors, collies and poodles.

The findings promise to uncover the genes responsible for traits such as aggression, herding, retrieving and body size, as well as those that cause more than 300 inherited canine diseases. This would enable breeders to eliminate painful disorders such as hip dysplasia, which particularly affects varieties such as labradors and golden retrievers, by choosing only the healthiest dogs for breeding.

Genetic screening could also allow more targeted selection of positive traits, such as a placid temperament for guide dogs.

Human medicine will benefit as well, as at least half the inherited conditions that affect dogs resemble human diseases. A better understanding of the canine genome will thus help scientists to trace the genetic origins of cancer and other life-threatening disorders in people.

The study, led by researchers at the Fred Hutchinson Cancer Research Centre in Seattle, has also shed important light on the evolution of modern dog breeds. It shows that each breed belongs to one of four clear genetic lineages, reflecting the way that it has been isolated and selectively mated by human beings over many centuries.

The most ancient group includes dogs of African, Asian and Arctic origin such as the Siberian husky, Alaskan malamute, Afghan hound and pekinese. These show the closest genetic resemblance to the grey wolf, from which dogs were domesticated in Asia about 15,000 years ago.

The other genetic groups are the mastiff-like dogs, including the rottweiler, bulldog, boxer and German shepherd; the herding dogs, including the collie and old English sheepdog; and the hunting dogs, including the labrador, golden retriever, terrier, beagle, bloodhound and dachshund.

In the research, details of which are published today in the journal Science, a team headed by Elaine Ostrander, Leonid Kruglyak and Heidi Parker took cheek swabs from 414 purebred dogs of 85 breeds to obtain samples of their DNA. These were examined using a technique known as microsatellite analysis to draw up distinct signatures for each breed.

These proved so reliable that in a blind test the scientists assigned 99 per cent of dogs to the correct breed by looking at their genetic profile. Only four out of the 414 dogs were assigned wrongly, all to a very closely related breed.

“The first major finding was that the different breeds are quite genetically distinct,” Dr Kruglyak said. “The dogs of a particular breed are much more similar to one another than they are to different breeds. These differences are so distinct that we could just feed a dog’s genetic pattern into the database, and the computer could match it to a breed.”

While scientists have long understood that the differences between breeds were caused by genes, the team was amazed to be able to detect them so easily: it is impossible to distinguish between human races in this fashion.

“This finding was a bit surprising because most of the breeds are quite recent and were formally genetically isolated only in the 19th century, with the advent of breed clubs and breed standards,” Dr Kruglyak said. “It’s a much more striking difference than is seen among human populations that evolved on different continents.”

The way in which dogs, which all belonging to the same species, have so many distinct inbred traits make them an ideal animal model for investigating genetic origins of human disease.

Individual genes stand out much more obviously in populations closely related to one another, which is why genetic scientists often study relatively isolated human societies, such as that of Iceland.

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