Dogsey

From the smallest Chihuahua to the largest Great Dane, dogs dramatically vary in size, much more than most other creatures and an international team of scientists reports that it has found the reason is down to a scrap of DNA.

Ranging from toy to giant, dogs have the biggest size variation of any mammal and the team of US and British experts believe it is due to variations in one gene.

After analysing DNA samples from 3,200 dogs, they pinpointed IGF-1, a gene that makes a hormone called insulin-like growth factor 1.

Variants of the gene were very strongly associated with the different sizes of dogs and the scientists are now trying to determine the precise mutations occurring within the gene.
The scientists began by looking at Portuguese water dogs, which range from 25 pounds to 75 pounds. Small and large Portuguese water dogs had one region of their DNA that differed. It was here that the IGF-1 gene, which is known to influence body size in humans, was found. They then analysed the same region in hundreds of dogs from 14 small breeds.
The IGF1 gene's hormone helps humans and other mammals grow from birth to adolescence. Medium and large dogs have the IGF1 gene but lack the newly discovered piece of DNA. But in small dogs, this scrap of DNA next to the IGF1 gene suppresses the gene's activity, keeping small dogs from growing larger.