Great Danes stretch more than a meter from paw to shoulder and can easily weigh more than 90 kilograms. A Chihuahua fits snugly inside a purse. Domestic dog breeds are more varied in body size and shape—not to mention coat color and fur length—than any other land-based mammal. Yet, according to a new study, a mere two to six regions in doggy DNA account for most of this diversity. Over the past few years, researchers have linked a number of canine traits—from size to coiffure—to specific mutations in dog DNA. This new line of research was made possible by the completion of the Dog Genome Project in 2005 by the National Human Genome Research Institute (NHGRI) in Bethesda, Maryland. But researchers lacked a large-scale analysis of these traits across a wide variety of breeds. As a result, they didn't know whether traits were governed by a large number of genetic regions, each contributing a small effect, or by a few regions with large effects.
So a team led by Carlos Bustamante, a comparative geneticist at Stanford University in Palo Alto, California, and Elaine Ostrander, a comparative geneticist with NHGRI, analyzed genetic information from 915 domestic dogs representing 80 different breeds. The researchers compared the dogs' DNA, looking for sequences that differed by a single base, known as single-nucleotide polymorphisms. Once they found out where the DNA differed, they compared those differences between dogs with, for example, short versus long legs or perky versus droopy ears. All told, the researchers identified 51 regions in the genome that contributed to physical variation among the breeds. These regions can be clumped into larger areas of the genome called quantitative trait loci, which are known to contain genes that produce a specific physical effect, such as shaggy hair. Depending on which traits are compared, genetic differences in two to six of these regions—which include genes, many of which haven't yet been mapped to specific traits—can account for about 80% of the variation in physical characteristics among dogs, says Bustamante. That differs significantly from humans, he says, whose physical variation is scattered far more widely across their genome, often comprising hundreds or thousands of regions.