One Gene Determines Size of Dogs

Scientists have just discovered which gene fragment controls the size of dogs, which have the greatest size range of any mammal — no other species produces adults with 100-fold differences, like that between a two-pound chihuahua and a 200-pound Newfoundland.

In a study published in the journal Science, researchers analyzed 3,241 purebred dogs from 143 breeds. Genetically, tiny dogs are almost identical to large dogs, except for a tiny bit of DNA that suppresses the “insulin-like growth factor 1” (or IGF-1) gene.

Dog breeders have unwittingly been selecting for the gene since the last Ice Age. Dogs emerged from the wolf about 15,000 years ago, and as far back as 10,000 years ago, domesticated dogs as big as mastiffs and as small as Jack Russell terriers were trotting the earth.