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The Small-Dog Gene

We now know why a dog can be as small as a gerbil or as big as a tiger–and it says more about us than it does about our pooches.

I have always been a big-dog man–until my girlfriend brought home Brownie, a 12-pound Jack Russell terrier. He thinks he is the size of a horse, and he’s as smart and nutty as any of the big Labradors I’ve had. We have often marveled at how a dog with a head and brain the size of my fist can be a real dog.

Credit: Technology Review

Now we know, thanks to Nathan Sutter, a geneticist at the National Human Genome Research Institute (NHGRI), in Bethesda, MD. Sutter and his colleagues tested thousands of dogs from 143 breeds, ranging from the tiny to the gigantic. They discovered that the smaller dogs have a distinct pattern of DNA in the growth-promoting gene IGF-1; they eventually narrowed down the difference to a single SNP–a single nucleotide polymorphism, which is a single base pair of DNA that differs among individuals of the same species. The SNP occurs in the middle of the gene just in the smaller dogs.

The interesting part is that we humans appear to be responsible. All 400 breeds of dog have been traced to a common gray-wolf ancestor living about 15,000 years ago–which humans domesticated. Our ancestors then became genetic engineers by breeding early pooches for various traits. According to an article about Sutter’s study in the current issue of Science,

The variant appears in so many small breeds, many dating back centuries, that it must have arisen early in the history of dogs, the authors argue. It may have become widespread because humans tended to select for more compact animals that were easier to feed, house, and transport, says co-author Robert Wayne from the University of California, Los Angeles. Just how this IGF1 variant controls size remains unclear.

As we contemplate more radical tinkering with genes in a variety of species, let’s keep in mind that our long history of manipulating man’s best friend has created both cuddly-cute pets and monster dogs bred for war and killing. This makes me wonder if there is an attack-dog SNP yet to be discovered–or a cuddle-up gene.

We humans are a strange species, having created a dog for every mood and attribute of our own species over the past 15 millennia. Perhaps we should keep this in mind as our technological prowess allows us to make ever-greater genetic manipulations.

Pennie, Elizabeth, “Roll over! Fetch! Don’t Grow Up!”, ScienceNOW Daily News 5 April 2007.

Sutter, Nathan B., et al., “A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs,” Science, 6 April 2007, Vol. 316, no. 5821, pp. 112-115.

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