Call it the "yappy" code.
An international team of scientists has identified a piece of dog DNA that seems to be the difference in the tall and the short when it comes to canine stature.
The genetic snippet regulates a particular gene. And the two are found together in all breeds of small dogs, but not in medium and large breeds -- except for rottweilers. More on them later.
The gene itself is found in all dogs -- all mammals, in fact -- and is crucial to pumping out hormonal growth signals from birth through adolescence. It appears likely that the same kind of genetic restriction also plays a role in limiting size in humans.
"Nearly all of what we learn from studying body structure, behavior and disease susceptibility in dogs helps us understand some aspect of human health and biology," said Elaine Ostrander, senior author of the study published today in the journal Science. She is also chief of cancer genetics at the National Human Genome Research Institute.
"By learning how genes control body size in dogs, we are apt to learn something about how skeletal body size is genetically programmed in humans," she added. "We also will increase our data set of genes likely to play a role in diseases such as cancer, in which regulation of cell growth has been lost."
The gene in question is called insulinlike growth factor 1 (IGF-1). The genetic quirk, called a haplotype, is found in all small breeds, from Chihuahuas and to Yorkies and Pekinese.
"All dogs under 20 pounds have this, all of them. That's extraordinary, " said Gordon Lark, a biologist at the University of Utah. Lark, with Utah colleague Kevin Chase, got the study rolling with a look at size variation among Portuguese water dogs, which can range in size from about 25 pounds to more than 75 pounds.
The researchers analyzed DNA samples and reviewed X-rays of more than 500 Portuguese to narrow the size regulator down to one stretch of code on dog chromosome 15.
Researchers from other universities and pet-industry labs in the United States and England joined in, bringing different tools. Ultimately, the analysis included more than 3,200 dogs from 143 breeds.
Dogs evolved from wolves roughly 15,000 years ago as humans domesticated them. Because the small-dog coding was found in small breeds that are only distantly related, and in different regions, the researchers figured the variant must be at least 12,000 years old.
"It's as ancient as all small dogs," Lark said. "Dogs are derived from wolves. Since this is found in all small dogs, it either got into dogs when they were first domesticated, or it was a small wolf that dogs descended from. The small-dog haplotype is not found in wolves today."
Of course, there's an exception to the rule.
Rottweilers, hardly lapdogs, also carry the sequence for smallness. The researchers said that obviously there are other genes involved in size when in comes to rottweilers, and probably other large breeds.
Lark and his colleagues figured that the genetic signal to make small dogs arose either because "a small wolf couldn't survive in nature, but it could survive in company with humans," or because early humans "wanted to domesticate a wolf, and they didn't want to adopt a big sucker" for reasons of safety, economy or crowded conditions in early walled settlements.
It may be simply that the "aww" factor tugged at prehistoric human hearts as much as it does today.
"Everybody treats their dogs like their babies, so it's not surprising they would select for tiny dogs," said Chase, the owner of two toy poodle/Maltese mix mutts. "Tiny dogs are not particularly functional. They don't hunt with you. They don't protect your house. They don't pull carts. They're just small and sweet."
And very often yappy, Chase concedes. "But 'yappy,' we didn't study."




children, but if you want a little tiny one, don't be afraid because of what someone who may not have all the facts says about them.  Be prepared to pay more (sometimes allot more) for the little guys though.  The breeders paid more for the parents.  They typically only have one or two pups in a litter compared to a Standard Size average of 5 or 6.  Because of their tiny size, they need to be with their momma's longer (until they are at least 2 lbs) so their is more time and expense involved before they can go to their new families.  Where a Standard Size will easily be 2 to 3 pounds or more at six to eight weeks old, a very tiny Imperial may not reach 2lbs until they are 3 or 4 months old. 
Canine Size
Why are small dogs small? Science thinks it knows
By LEE BOWMAN
SCRIPPS HOWARD NEWS SERVICE
What Makes a Small Dog Small?
Scientists think they have the answer.
           Kali... A Small Dog  
3 pounds 15 ounces at 10 months old
    I have seen many web sites where well meaning breeders of "standard" size shih tzu post false information regarding the Imperial size dogs.  I want to let you know that the myth that the smaller dogs are "runts", "unhealthy" or of "inferior quality" is UNTRUE.  I know of many many breeders of the smaller size Imperial and their dogs as well as mine are happy, very healthy little dogs with beautiful coats. These little dogs are every bit as healthy and are beautifly quality dogs in a tiny little package.  These cute little bundles of fur are not new and many good breeders have been breeding them for as many as 20 years.  This is not to say that you should not do your research and ask plenty of questions when considering buying ANY dog.  This picture is only one example of a 6 month old Imperial.  As you can see, she has clear eyes, a beautiful coat, clear bright eyes is well proportioned, no hernia and she has no health problems.  My vet adores her as well as my 4.5 lb male Ripley.  If you are looking for a Standard Size Shih Tzu, they are wonderful dogs with awsome temperments, and generally very good with