An international team of scientists, including researchers from Cornell University, has found a mutation in a single gene that plays a key role in determining// body-size differences within and among dog breeds and probably is important in determining the size of humans as well.

The research, published in the April 6 issue of the journal Science, "is one of the first demonstrations that if you look at different dog breeds that share the same trait (such as large or small size or short legs or scrunched-up faces) due to human-directed selection, you will find genes that are likely to affect the same traits in humans," said co-author Carlos Bustamante, assistant professor of biological statistics and computational biology at Cornell.

The researchers began by comparing the DNA of individuals within a single dog breed that shows great variation in skeletal size -- Portuguese water dogs -- and identifying regions of the genome that differ between small and large individuals.

One of these regions included a gene that codes for a protein hormone called insulin-like growth factor 1 (IGF1), which is known to play an important role in growth, body size and longevity in mice and in body size in humans. The researchers then analyzed this region in hundreds of dogs from 14 small dog breeds, such as Chihuahuas, toy fox terriers and Pomeranians, and nine large dog breeds, including Irish wolfhounds, Saint Bernards and Great Danes.

Surprisingly, almost all of the dogs from the small breed shared a stretch of DNA that was identical to that found in small Portuguese water dogs, but different from that found in large breeds, large Portuguese water dogs, wolves and other wild members of the dog family, such as jackals.

Researchers suspect a single mutation early in the domestication history of dogs created a "small dog" version (or allele) of IGF1. This mutation became fixed in small dog breeds through human breeding of the dogs.
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