All dogs are members of the same species, and each characteristic breed is a result of selective breeding by humans. Now, scientists have found that the extreme differences in dog size – between say a Chihuahua and Great Dane – are largely determined by a single change in a single gene.

“A Single IGF1 Allele is a Major Determinant of Small Size in Dogs” by Nathan B. Sutter and others, Science (volume 316), April 6, 2007, pages 112-115.