From BBC News:
Researchers studied 3,000 dogs from 143 breeds and found small dogs all shared a mutation in a gene that influences size in other animals. This form of the gene was almost absent in large dog breeds, an international team reported in Science journal. The 14 small dogs in the sample, such as Portuguese water dogs, chihuahuas, fox terriers and pomeranians, share a specific sequence of DNA that includes the gene for making a hormone called insulin-like growth factor 1 (IGF-1). The scientists also looked at DNA from nine large breeds including Irish wolfhounds, St Bernards and Great Danes.
The IGF-1 gene has been known to influence size in other organisms, including mice and humans. These results suggest that while there are invariably differences among breeds – even in genes for size – IGF-1 has played an important role in the evolution of many small breeds by being a gene that consistently affects body size. The new research suggests that a mutation in this gene led to the appearance of small dogs more than 10,000 years ago.
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