Depending on how much time is attributed to a generation and how many generations are involved, scientists can determine breed age. Although closely related to one another (some more closely than others), breeds can be identified as distinct based on the way their DNA segregates, or separates during gamete formation. Data suggest that the most ancient breeds are dogs that looked similar to modern-day Basenji, Saluki, Afghan Hound, Tibetan Terrier, Lhasa Apso, Chow Chow, Pekingese, Shar-Pei, Shih Tzu, Akita, Shiba Inu, Alaskan Malamute, Siberian Husky and Samoyed. All the others — from Affenpinscher to Slovakian Rough-haired Pointer to Yorkshire Terrier —were created in the last 400 years.
Scientists suspect that many foundation traits differentiating ancient dogs evolved only once. Perhaps they were embedded in the genome of the nowextinct wolf species that begat domestic dogs. For instance, achondroplasia (short-limbed dwarfism), a defining trait in 19 breeds, including Dachshunds, Corgis and Bassett Hounds, is the result of a single evolutionary event (Parker et al. 2009). This means that mongrel dogs with short limbs are not necessarily mixes of short-limbed breeds. Rather, it’s the other way around: short-limbed dogs were accompanying our ancestors long before Doxies and Corgis were engineered less than 400 years ago.
Brachycephalia (disproportionate shortening of the muzzle) is another foundation trait. A Boxer wouldn’t be mistaken for a Pug, nor a Bulldog for a Pekingese, but they all share brachycephalic head types. Archeological evidence from ancient gravesites indicates that brachycephalia existed long before the formation of modern breeds. And indeed, a single genetic variation causes this trait, no matter how different the breeds look (Bannasch et al. 2010).
The challenge comes in teasing out the handful of genes associated with these foundation characteristics when the breeds that share them are vastly different. To do so, researchers devised a method to map dog traits developed under extreme selection: find the marker in a single breed and it will provide a clue as to where it will be found in all dogs. Because size differences are magnified in certain breeds, looking for genes associated with size was a starting point. Information gleaned from archeological sites indicates that small stature in dogs appears to be an ancient trait.
Diminutive breeds can be as small as six pounds, whereas some giants weigh in at just under 200 pounds, and height can vary from less than six inches to as much as three feet. Even so, all puppies are born almost the same size, with short muzzles for nursing and stout legs for pushing siblings out of the way at mealtime. Why does one turn into a calm 180-pound giant and the other into an excitable eight-pound lap dog?
Studying the genetic make-up of 526 dogs from 14 small breeds, including the Chihuahua, Toy Fox Terrier and Pomeranian, and nine giant breeds, including the Irish Wolfhound, Saint Bernard and Great Dane, scientists pinpointed a specific gene-sequence variant in the canine genetic code associated with small size (Sutter et al. 2007). They concluded that a single gene that encodes an insulin-like growth factor is common to all small breeds and is nearly absent from all giant breeds, implicating it as a major influence on small stature in dogs. But there are exceptions. Though at some stage of early growth, all little dogs get the genetic memo to stay small, some dogs, like Rottweilers, are born with the small-stature variation but grow large in spite of it, indicating that in certain cases, something overrides the message associated with small size.
This is not to say, however, that a single gene controls size. In genetics, big is not the opposite of small. In other words, a dog who doesn’t carry the small gene is not necessarily destined to be big. Researchers found several other gene variants that affect size to various degrees; large size is linked to three of them.