Benjamin Lester of Cosmos Online reports:

Much of the amazing variation in size of different breeds of domestic
dogs stems from tiny differences in a single gene, say researchers.

When the team compared the dog DNA samples, they found that all the
small breeds shared the same mutations in a small part of the DNA of
one gene which regulates the production of insulin-like growth factor
(IGF1) - a protein key to promoting growth. . . .

"In the big dogs, the [level] of insulin like growth factor is
larger than in the small dogs," said Alan Wilton, an expert in dog
genetics at Sydney’s University of New South Wales, who was not
involved in the study.

The result, said the researchers, is that dogs with this mutation
stay small. "All dogs under nine kilograms have this – all of them," said Lark. "That’s extraordinary."

Image: Deanne Fitzmaurice