When Will Colosimo adopted his dog Allie in 2003, he knew he was getting a mutt. She looked like a basenji, but the Colorado Basenji Rescue group in Denver, from where he retrieved her, said they didn't think she had any of that small, short-haired breed in her.

Curiosity got the better of him.

"We always knew she was beautiful, but we didn't know what all came together to make her," he said.

There are several types of DNA tests available for determining a mixed-breed dog's ancestry. Colosimo sent away for one that required swabbing the inside of his dog's cheek and mailing the sample to a lab. He learned that Allie, who is 8 or 9 years old, had both German shepherd and dachshund blood.

"It was hilarious," said Colosimo, 45. "So, the German shepherd I can totally see, but dachshund? That's crazy."

And not uncommon. Veterinarians advise owners that what they see in their dog is not always what they've got.

"We're really bad guessers at what dogs are," said Martha Smith, Director of Veterinary Medical Services at the Animal Rescue League of Boston.

The rescue league began using mixed-breed DNA blood testing when it appeared about four years ago, testing a few of its shelter dogs. "We found out from the handful of tests that we ran that we were way off base" in guessing breeds, Smith said. The test "proves dogs are individuals."

Karin Hendersin, 52, a market researcher in Denver, can speak to that. Her dog, Splash, resembles a pit bull, a breed banned in the Denver city limits. Hendersin recently learned that Splash, with her brown-brindled coat, is Chinese Shar-Pei, Labrador retriever and Dalmatian — and no pit bull.

Hendersin thinks the DNA test also helped explain some of the dog's behavior.

"It explains why she's such a runner," Hendersin said, noting the Dalmatian genes. "We take her to the dog park and a whole herd of dogs will chase her."

There are two kinds of mixed-breed DNA testing: the inner-cheek swab method, which is a kit that can be bought at stores or online, and a blood-drawn test, which is performed in a veterinarian's office.

The cheek-swab method, created by MetaMorphix Inc., a biotechnology company, is offered at two levels: The standard breed test (about $70) can identify from a range of about 50 dog breeds, while the "XL" breed test (about $120) identifies from about 100 breeds.

The blood test identifies genes from a base of 157 breeds, according to the Web site of Mars Inc., the company that offers the test through vets' offices. It costs about $200 but includes a veterinary appointment for reviewing the results.

Smith advocates for the blood-work test not only because it accesses more breeds but because, depending on the DNA results, some dog owners may need follow-up counseling.

"Some people might've been happy with what they thought they had, then something like 'Rottweiler' shows up," said Smith. "All of a sudden, they're looking at their dog through a completely different pair of eyes."

A veterinarian can reassure owners that "the dog you now have more information about is the dog you still love," she said.

Theresa Brady, a MetaMorphix marketing representative in Calverton, Md., said the two DNA tests are equally effective, even though her company's cheek-swab method tests for fewer breeds.

"DNA is DNA," she said. "The sampling method doesn't make a difference."

Smith and other vets caution that the DNA tests are "for fun and entertainment" — not for diagnostic purposes.

"It's just a test for owners," said vet Kelly Best of Arvada Flats Veterinary Hospital, in Colorado. "I don't know that it has any medical benefits at all."

Even for purebreds predisposed to certain diseases, their genetic dilution in a mutt makes concern about the diseases negligible, she said.

And no one has come knocking on her door asking for the test.

Smith, however, has run the blood test on many dogs.

"It's like Christmas day when (clients) get to open their results," she said. "A lot of times people are right, and a lot of times they're wrong."

Either way, "people are really, really excited," Smith said.