Paw prints: how scientists use DNA to learn about the past, present, and future of dog breeds.

What's a bagel? If you answered ring-shaped bread, you re only partly correct.

A bagel is also a type of dog--a cross between a basset hound and a beagle. This mixed-breed dog with floppy ears and short legs may not be prized as a household pet yet. But if demand for bagels increases and its population swells, the dog may be recognized as an official breed, or a group of organisms descended from common ancestors.

For centuries, humans have selectively coupled different dog breeds. The goal: to create a new breed that carries the most desired traits, or inherited characteristics, of both its parents (see Nuts & Bolts, p. 11). For example, a breeder might aim to produce a dog featuring the curls of one breed, and the roly-poly body of another breed. With so many possible combinations, there are already close to 400 recognized dog breeds--each with unique characteristics.

But fluffy or furless, jumbo or squat, all dogs belong to the same genus and species: Canis familiaris, or the domestic dog. Of all species of mammals, or animals that nurse their young with milk, "dogs probably have the greatest range of morphologic (pertaining to physical shape and size) variation," says Elaine Ostrander, a geneticist who studies heredity, at the National Human Genome Research Institute (NHGRI).

How did a single species produce members with such a wide array of physical features? And where did dogs come from to begin with? Scientists are examining dogs' DNA, or the chemical that carries hereditary information, to learn how the species has evolved.

PUPPY TALE 

All dogs have 42 teeth and are naturally pack animals, just like their canine relatives--wolves, coyotes, and jackals. For a long time, scientists wondered which one, or combination, of these historically older, wild relatives gave rise to dogs. In 1997, Robert Wayne, a geneticist at the University of California at Los Angeles, conducted a DNA study to answer the question.

What makes DNA a good research tool for tracing dogs' ancestry? DNA contains the blueprint of life. Every individual organism, except for identical twins, has a unique set of DNA. And since DNA contains hereditary information that is passed from parents to offspring, the closer the organisms are related, the more similar their DNA.

Wayne isolated the DNA from the hair, blood, and tissues of several dog breeds. Then, he compared the DNA with that of wolves, jackals, and coyotes. It turns out that dogs and wolves have by far the closest genetic match. So Wayne concluded that dogs must have descended from wolves. "The earliest dogs may have even looked just like wolves," he says.

DOG TRAINING

But an ancient pack of wolves didn't suddenly turn into domestic dogs overnight. Scientists think that dogs--the world's first domesticated, or tamed, animal--originated in Southeast Asia between 15,000 and 40,000 years ago. "The domestication process was probably long and involved many different wolf populations," says Wayne.

During the time when wolves first began evolving into dogs, humans wandered Earth and lived as hunters and gatherers. Scientists believe that some wolves followed humans around because their food scraps provided an easy meal. At the same time, some humans may have raised abandoned wolf pups. As the relationship between humans and early-dogs grew, the animals began defending people and aiding them in hunts. Eventually, humans started breeding tamed canines that had desired traits, such as loyalty and good hunting abilities.

As time passed, humans and their dogs began to migrate to various parts of the world (see map, p. 9). As humans settled in vastly different areas and developed civilizations, their companion dogs adapted. They gradually changed to fit in better with their new surroundings. For example, dogs in warmer environments may have developed shorter hair. "Perhaps some dogs became more docile because they had to live in close quarters or get along with livestock," says Wayne. These new traits were then passed on to future generations of pups.

MIX-AND-MATCH POOCHES

Throughout history, dog breeds have fallen in and out of popularity. To suit these trends, dog breeders regularly create new breeds. Roughly 80 percent of the nearly 400 dog breeds were developed in the past 300 years.

Today, the dog species has a mind-boggling variety of members. "If we were to line up the tallest and shortest of humans, we would notice their differences, but not by as much as the range we see in dogs," says Ostrander. In dogs, "it's astounding that members of the same species can have legs ranging from 6 inches to 2 feet long, or body sizes that vary as much as 40-fold."

To understand how the dog species became so physically diverse, Ostrander is heading the Dog Genome Project at NHGRI. "We're trying to figure out at the DNA level what it means to be a dog," she says.

FAMILY TREE

For the project, Ostrander's team first used cotton swabs to collect cheek cells from several dog breeds. From these samples, they isolated the dog's DNA. By comparing the DNA data, the team is learning how each dog breed is related to the others.

Ostrander says the process is similar to comparing the unique ridges from different humans' fingerprints. "Closely related breeds--the mastiff and bullmastiff, for instance--have fewer points of variation in their DNA fingerprints," she says. So far, the project has divided 85 dog breeds into four populations that reflect similarities in morphology and geographic origin.

The scientists also hope to use the DNA data to learn the origins of specific dog features. For instance: "We want to know what makes big dogs big, and little dogs little," says Ostrander. By zeroing in on the DNA segments that closely related dog breeds have in common, the scientists hope to track down the genes, or units of hereditary material, that gave rise to shared traits, such as size, a pointy nose, or even wrinkly skin.

In addition, by learning how dog breeds relate through their DNA, scientists may find ways to improve canine health. Similar to humans, dogs can inherit certain diseases, including cancer, from their parents.

If closely related dog breeds have a disease in common, "then I can assume that there's a good chance that they inherited a common ancestral mutation (variation in DNA)," says Ostrander. By pinpointing these mutant genes, scientists will be better able to find ways to help dogs beat the diseases.

DOG TAGS

A history of good health is one factor that the American Kennel Club (AKC), a canine authority, looks for when deciding if a mixed-breed dog should be declared an official breed in the U.S. The AKC also requires proof of the mixed-breed's popularity, and records of ethical breeding practices. In 2004, the AKC recognized three new dog breeds, including a Neapolitan mastiff (see photo, p. 10, top).

Will the bagel, labradoodle (labrador retriever-poodle mix), goldendoodle (poodle-golden retriever mix) or even the cockapoo (cocker spaniel-poodle mix), ever qualify as breeds? For many owners of these mixed-breed dogs, that doesn't matter. The only stamp of approval needed is the one between dog and owner.

Nuts & Bolts

Organisms usually have two copies of most genes-one copy from each parent. The different forms of a gene are called alleles. To help determine the odds that an offspring will have a certain trait, you can use a Punnett square:

1 Given each parent's genetic makeup, assign an uppercase letter for a dominant allele (one that expresses a visible trait) and a lowercase letter for a recessive allele (one that is masked by a dominant trait).

2 Put one parent's letter pair across the square's top, and the other along the left.

3 Cross, or match the top and left letters, to fill in the inner squares. These show the offspring's allele pairs.

DID YOU KNOW?

* There are nearly 400 clog breeds recognized by various canine authorities around the world. The American Kennel Club, however, recognizes only 153 of the breeds.

* According to dog registration information in the United States, the most popular dog breed in 2004 was the labrador retriever, with 146,692 registered canines. It was followed in order by golden retrievers (52,550), German shepherds (46,046), beagles (44,555), and Yorkshire terriers (43,522).

CRITICAL THINKING:

* Each country's canine authority has its own set of criteria for determining if a dog should be considered a registered breed. Suppose you were to set up a new canine authority. What standards must a dog meet in order to be recognized as an official breed by your organization? Explain your reasoning.

CROSS-CURRICULAR CONNECTIONS:

MATH: Have students do research to create a budget projection on how much it would cost to take care of a dog for one year. Remind students that besides food, they will need supplies (leashes, dog bowls, etc.) for their pets. Also, (togs need medical care, and some breeds require regular grooming.

RESOURCES

* Read about the American Kennel Club's criteria for recognizing dogs as registered breeds: www.akc.org/press_center/facts_stats.cfm?page=8

* For more information about the Dog Genome Project, visit: http://research.nhgri.nih.gov/dog_genome/

(PAGE 8) PAW PRINTS

DIRECTIONS: On a separate piece of paper, answer the following in complete sentences.

1. Name three of dogs' canine relatives. What features do canines share?

2. Why is DNA a good research tool for tracing ancestry?

3. When, where, and how did wolves first evolve into dogs?

4. How might understanding how different dog breeds relate through their DNA help scientists find ways to improve canine health?

ANSWERS

1. Dogs' canine relatives include wolves, coyotes, and jackals They all have 42 teeth and are naturally pack animals.

2. DNA contains the blueprint of life. Every individual organism, except for identical twins, has a unique set of DNA Since DNA contains hereditary information that is passed from parents to offspring, the closer the organisms are related, the more similar their DNA

3. Scientists believe that dogs--the world's first domesticated animal--originated in Southeast Asia between 15,000 and 40,000 years ago. During the time when wolves first began evolving into dogs, humans wandered Earth and lived as hunters and gatherers, Scientists believe that some wolves followed humans around because their food scraps provided an easy meal. At the same time, some humans may have adopted and raised abandoned wolf pups. As the relationship between humans and early dogs grew, the animals began defending people and assisting them in hunts Eventually, humans started breeding tamed canines that had desired traits, such as loyalty and good hunting abilities.

4. Similar to humans, dogs can inherit certain diseases, including cancer, from their parents. If closely related dog breeds have a particular disease in common, then there's a good chance that they inherited a common ancestral mutation. By finding these mutant genes, scientists will be better able to find ways to help dogs beat the diseases.