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February 16, 2009

How Weiner Dogs Got Their Funny Legs

DachshundThe Scottish terrier that was a finalist at the Westminster Dog Show last week is a mutant. And she's not alone. With their stubby legs and long bodies, Scottish terriers, Bassett hounds, and dachshunds have been purposely bred by humans to have chondrodysplasia, a dwarfing of the legs that is considered a defining--and desirable--trait in some breeds of little dogs. Now, geneticist Elaine Ostrander of the National Human Genome Research Institute in Bethesda, Maryland, and her colleagues have identified the underlying genetic variant that truncated these dogs' legs and made it possible for dachshunds, for example, to burrow down a hole to hunt badgers.

As she showed glossy images of her photogenic canine subjects, Ostrander described how researchers have been seeking genes that explain the origins of diversity in the 300 known breeds of dogs, which come in all shapes and sizes, with long hair and short fur, smashed faces and long snouts. In 2007, Ostrander's lab found a single gene in short dogs, such as Chihuahuas and Pekingese, that regulated the expression of insulin-like growth factor 1 (IGF-1); the gene was missing in giant breeds, such as bullmastiffs and great Pyrenees.

So what about those funny legs?

Ostrander's lab has homed in on the "short" gene in detail and found a new surprise: Not only does it make dogs small, but some dogs carry an extra genetic element inserted by chance into chromosome 15. The element--a so-called retrogene--changes the expression of IGF-1 and gives the dogs the distinctive short, bowed forelimbs of chondrodysplasia. "This is fascinating," says geneticist Sarah Tishkoff of the University of Pennsylvania in Philadelphia, who studies genetic diversity in human evolution. 

The retrogene is found in so many of these small dogs that Ostrander thinks it inserted into the genome of a common ancestor before the dogs separated into 20 different breeds or so. The discovery is not just for the dogs, either--Ostrander speculated that a similar genetic mechanism might be responsible for some types of dwarfism in humans and may even play a role in cancer.

--Ann Gibbons