Scientists have decoded the dog genome. A boxer named Tasha, selected for her high degree of inbreeding, has had her genetic secrets puzzled out and then compared to partial genetic pictures of other breeds of dog and other mammals. It has been a long wait for humanity's first companion, domesticated from gray wolves at least 15,000 years ago.

Researchers first broke up Tasha's genome into small sections of genetic material, deciphered the makeup of each of those bits, and then pieced them together into a complete genetic map--the so-called whole genome shotgun strategy. Tasha's high degree of inbreeding simplified the task of decoding her 2.4-billion-letter genetic code by reducing the differences between her 39 chromosome pairs. The fact that she is a female, however, left the team without a picture of the canine Y chromosome.

The dog apparently shares 94 percent of its genetic sequence with humans and mice, despite having a substantially smaller genome thanks to fewer repeat sequences. And dog lovers will be happy to know that humans have more in common genetically with dogs than with mice, despite sharing a more recent common ancestor with the latter.

By comparing Tasha's DNA to that of nine other dog breeds, coyotes and four types of wolves, the researchers also found that all modern breeds--from poodles to malamutes--derive from at least two population bottlenecks: one that occurred thousands of years ago when humans bred dogs from wolves and one that happened in the last few hundred years as humans bred that ancestral population into the dogs of all shapes and sizes observable today.

Because each breed represents an isolated group with discrete traits that can be linked to distinct genes--and because different breeds suffer from some of the same maladies that afflict humans, such as allergies or certain types of cancer--the dog's genome should help isolate the genetic roots of such diseases, proving dogs' utility to humanity once more. "For millennia, dogs have accompanied humans on their travels," the researchers write in the paper presenting the findings in the current issue of Nature. "It is only fitting that the dog should also be a valued companion on our journeys of scientific discovery."