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.
GUY RAZ, HOST:
About 300 years ago, so the story goes, German hunters were looking to create a very particular kind of dog for a very particular kind of mission. The result? The lovable Dachshund. In a recent article for National Geographic, writer Evan Ratliff decided to investigate the origins of modern day dogs including the aforementioned Dachshund.
EVAN RATLIFF: These German hunters wanted some sort of dog to hunt badgers and other sort of small rodents that live in holes. So they wanted to create a small dog that was also long. And, you know, there's not real clear documentation like you would find now with a new breed, but this sort of preponderance of evidence that people cite shows that these breeders combined something like a hound, a Bassett hound with a terrier. So, you know, that was sort of getting the stumpy legs and getting the hunting capability.
And then they also bred for the sort of loose fur that Dachshunds have where if they were bitten it wouldn't actually cause them a great deal of damage. And then the sort of last component was this sort of long, very sturdy tail because once they chased a badger into a hole, you could actually pull them out by grabbing their tail.
RAZ: That's amazing. I had a Dachshund when I was little, and they used to chase squirrels very forcefully. And now it makes - it all makes sense.
RATLIFF: Yes, indeed.
RAZ: Very cute dogs, by the way.
RAZ: Extremely cute dogs.
RATLIFF: I grew up with miniature Dachshunds...
RAZ: Oh, even cuter.
RATLIFF: ...which was a further development along the advanced Dachshund line.
RAZ: OK. The Chinese shar-pei. How did they make that?
RATLIFF: You know, some of these breeds, it's just a matter of accentuating traits. Some, it's a matter of mixing traits, and others, it's a matter of accentuating traits. So, you know, in the case of the shar-pei, it's more about finding this sort of look and then pursuing the look. So breeding the dogs that look most like what you want.
RAZ: Now, one thing you write in the article is that for years, scientists just assumed that dogs were just as genetically complex as humans, but it turns out that they're not, actually, right? In some cases, it's a matter of flipping one or two switches and you change a dog's appearance.
RATLIFF: That's what's so incredible about the research that these particular scientists have done over the last - just over the last five or 10 years.
RAZ: This is a project called CanMap that you're talking about.
RATLIFF: That's right. And it's scientists working together from Cornell, UCLA, now also Stanford, the NIH, and what they're doing is they're sort of looking at dogs broadly. So they took a whole large collection of dogs - 900 dogs from I think 80 breeds - and they collected their DNA and they also collected all their traits, and then they put them all into a database and they started comparing them.
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, so sometimes between one and five gene regions.
RAZ: You describe the makeup of dogs as Tinkertoy genetics, essentially very simple. Why is that? Why are they so simple?
RATLIFF: Well, it has to do with how these breeds were created. So, you know, in nature, the way that natural selection works, it usually works on very, very small changes. In fact, large changes in animals in nature can be bad. But in dogs, because humans were controlling the process, they tended to say, I want a much taller dog. And so they would emphasize that or even bring in a different breed - two breeds - and put them together.
And the effect of that is that those kinds of changes act on genes with very strong impacts. So if I want a tall - a large dog, then I end up selecting for this gene called IGF1 which has a very, very strong effect on the size of the dog. And when you do that over a couple of hundred years, what happens is it turns out that IGF1 gets selected over and over and over again so it becomes the gene that controls body size. So now in dogs, if you know what IGF1, what the value is for a dog, you can predict their size about 50 percent.
RAZ: That's writer Evan Ratliff. His article in the latest issue of National Geographic' is called "How to Build A Dog." Evan Ratliff, thank you so much.
RATLIFF: Thanks for having me. Transcript provided by NPR, Copyright NPR.