Almost every physical trait in dogs is controlled by just a few genes

From NPR:

For years, scientists thought that dogs were just as genetically complicated as humans, Ratliff says. But that turned out not to be the case. Scientists at Cornell, UCLA, Stanford and the National Institutes of Health have been comparing dog DNA as part of a project called CanMap.

"They took a whole large collection of dogs, 900 dogs from, I think, 80 breeds," Ratliff says. "And what they learned was that in these dogs, if you look at their physical traits, everything from their body size to their coat color to whether they have floppy ears, it's determined by a very small number of genes."

It's actually human interference that's the cause of what Ratliff calls "Tinker-Toy genetics" in dogs. "The way that natural selection works, it usually works on very small changes," he says. Sudden large changes can actually be harmful.

But breeders can introduce large changes in a dog relatively rapidly, by selecting the genes that have the strongest effects.

"If I want a tall dog, a large dog, then I end up selecting for this gene called IGF1, which has a very very strong effect on the size of a dog. And when you do that over a couple of hundred years, what happens is ... it becomes the gene that controls body size."

More here.