From the smallest Chihuahua to the biggest Great Dane, all dogs emerge from the same basic set of genes.
Scientists announced Wednesday that they'd deciphered this doggie DNA and begun detailed comparisons between breeds and with humans.
The work should help researchers find genes that make dogs and people vulnerable to such illnesses as cancers, heart disease, cataracts, epilepsy, blindness and deafness. And it has already hinted that people may have fewer genes than scientists thought.
The work used the DNA of a female boxer named Tasha, said Eric Lander, director of the Broad Institute of Harvard and MIT and senior author of the analysis in Thursday's issue of the journal Nature. The DNA project was led by Kerstin Lindblad-Toh of the institute.
Tasha was chosen from more than 100 candidate dogs because her DNA looked especially amenable to the task of identifying its 2.4 billion chemical building blocks. But it turned out that any other dog would have worked just as well, Lander said.
The results are more complete than those announced in 2003 for the DNA of a male poodle named Shadow.
Scientists have also deciphered the DNA of mice, rats, chimps, chickens and, of course, humans, as well as many other organisms.
At the DNA level, two randomly chosen dogs differ by only about as much as two randomly chosen people do, yet the variation in appearance, size and behavior in dogs is "just mind-boggling," Lander said.
"How is it within one narrow gene pool you can produce Chihuahuas and Great Danes?" he asked.
Much of the answer involves differences in turning gene activity on and off, he said, and further study could help scientists understand what produces big differences in body plans between species.
The new work also identified signposts along the canine DNA that will help in finding genes that predispose dogs to certain diseases, some of which they share with humans. The dog genes should point the way to their human counterparts, Lander said.
In fact, it may be vastly easier to find disease genes in dogs than in people.
Intensive breeding has left its mark in the dog genome so that finding DNA regions that contain disease genes "is like hitting the side of a barn," Lander said.
Such genetic research should benefit dogs and their owners, said William Truesdale, a board member of the American Kennel Club's Canine Health Foundation, which had put $2 million into the dog DNA project.
"We're trying to erase these genetic frailties" by screening dogs for disease genes prior to breeding, he said.
That effort is now in its infancy, but as more of those genes are uncovered they can be eliminated by breeding dogs that don't carry them, he said.
And puppies can be tested to assure their owners that they won't get the associated diseases later, "like a Good Housekeeping seal," Truesdale said.
Dog DNA is already teaching several lessons about human DNA. For one thing, comparisons between DNA of dogs, humans and mice revealed elaborate controls on the activity of certain human genes active in early development, Lander said.
The three-way comparisons also showed that some genetic features found in humans but not mice aren't really unique to people, but also appear in dogs, he said.
"The more species we look at, the more, frankly, we find that humans are not exceptional here," Lander said.
Researchers also estimated that dogs have 19,300 genes, almost all of them canine versions of genes found in people.
Prior studies have indicated that people have about 3,000 more, but Lander said the dog analysis "is leading us to question whether those are in fact real human genes."
Some proposed human genes, he said, are now "suspect" and may not be genes at all.