Humans are complicated genetic jigsaw puzzles. Hundreds of genes are involved in determining something as basic as height.
But man's best friend is a different story. New research shows that almost every physical trait in dogs — from a dachshund's stumpy legs to a shar-pei's wrinkles — is controlled by just a few genes.
Writer Evan Ratliff has been looking into dog genetics for National Geographic Magazine. He tells weekends on All Things Considered host Guy Raz that that quirk makes it extremely easy for breeders to develop new, custom-designed dogs — like the German hunters who bred the original dachshunds a few hundred years ago.
"These German hunters wanted some sort of dog to hunt badgers and other sort of small rodents that live in holes." So they crossed long, low basset hounds with tenacious terriers, to produce a dog that could chase badgers into their dens and then be yanked out again by the tail if necessary. The breeders also built in loose fur, so any bites wouldn't do much damage.
Other breeds, like the shar-pei, developed after breeders pursued a particularly favored look, Ratliff says.
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."
No word yet on which genes control loyalty, dog breath, or a propensity to slobber on your slippers.