Chihuahuas, Boston terriers, and Pomeranians have this much in common: They’re tiny. Part of what makes them that way is the mutation of a single gene called insulin-like growth factor 1 (IGF1), according to a group of researchers from the University of Utah, Cornell University, and the National Human Genome Research Institute.
The researchers began their study by looking at dogs of one breed, the Portuguese water dog, and found that those with one type of mutation of IGF1 were 15 to 20 percent smaller. The researchers ultimately studied 3,000 dogs from 143 different breeds to determine how that gene mutation was distributed across the species. They discovered that the smallest breeds, like Chihuahuas, all had the same gene variant that would make them small. Similarly, 100 percent of the largest dogs, like Great Danes, had a variant that would make them big.
The group was surprised that so many small-dog breeds shared the same mutation. “It didn’t need to be that a gene that determines size within breeds would determine size across breeds, but that is how it turned out,” says Carlos Bustamante, an assistant professor of biological statistics and computational biology at Cornell, who crunched the numbers for the project. “Below a few kilograms, it’s staggering. More than 85 percent had the gene variant, about as ‘smoking gun’ of a correlation as we’ve seen.”
Domestic dogs are descended from gray wolves, which have only the big version of the IGF1 gene. Bustamante imagines that the small mutation probably arose around the start of domestication. “You had junky dogs living on the outside of settlements,” he says, “so a small mutation might be advantageous—you could get closer to a village without scaring everyone.” The researchers believe the mutation became fixed within different breeds during 300-odd years of artificial selection—that is, dog breeding.
However it arose, the switch is not limited to the Canidae family. Mice who’ve had that section of their genes knocked out wind up 40 percent smaller. And, scientists say, humans who share 90 percent of the amino acids found in small-dog IGF1 tend to be the more diminutive specimens of our species.
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