Scientists have discovered a genetic reason why some dogs are small and others big. They say the finding has implications beyond canine size, since the work applies to the study of other complex genetic traits, such as diseases and behaviors caused by multiple genes. VOA's David McAlary reports.

Think of the difference between a Chihuahua and a Great Dane.

No other mammal has as great a range of sizes as the dog does. The researchers now understand the genetic basis for that variation, and explain it in the journal Science.

What they found is a piece of dog DNA that regulates the activity of a nearby growth gene called IGF-1. Study leader Elaine Ostrander of the U.S. government's National Human Genome Research Institute near Washington says the researchers saw it first in Portuguese water dogs. Small members of the breed had one version of the gene and the large ones had a different one.

"That was interesting, but not shocking. What really blew our socks off is when we started looking at other breeds, we found that all the small breeds had the exact same genetic signature at IGF-1 as did the small Portuguese water dogs, and large dogs breeds had the same signatures as what we found in the large Portuguese water dogs," he said.

Ostrander and her colleagues began researching dog size because it was the easiest genetically complex trait to measure. They have not determined the exact mutation in the regulatory DNA that influences the growth gene, but she says finding the DNA region is a model for how to track down the genetic causes of other complex, multi-gene traits.

"If we can figure out how to do this in morphology, we can probably figure out how to do it in behavior as well, and understanding the vocabulary of growth and development gives us entrée into understanding disease," he said.

Another member of Ostrander's group, University of Utah geneticist Gordon Lark, says the scientists are now studying autoimmune diseases in dogs - diseases where the immune system attacks the animal rather than protects it, such as arthritis or allergies. He says the work could eventually lead to discoveries about the genetic basis for complex human ailments.

"Dogs have about 200 to 300 diseases in common with humans and they share the same environment. So for a very complex disease such as cancer, they are an ideal animal to study. And now that the dog genome has been sequenced, it can be compared directly to the human genome," he said.

Lark and Ostrander say they could not have carried out their study on dog size if it were not for the cooperation of hundreds of dog owners across the United States who sent in x-rays and DNA samples from their pets. Lark says it was not practical for them to house many different breeds.