Elegant and expressive, blue-eyed dogs have some of the most unforgettable furry faces around. Scientists have thrown dog-lovers a bone: clues as to what causes this eye-catching trait.
Scientists know that blue eyes are related to the coat color of some dogs, like merle (or splotchy-coated) Australian shepherds. But what about other dogs that sometimes have piercing baby blues, like the striking and popular Siberian husky? (Related: What Makes a Great Sled Dog? Breed, Ambition, Tough Feet.)
Scientists at a DNA testing startup set out to explore the question by looking into other variations—besides coat color—that might cause blue eye color in dogs.
In the largest study ever to compare dogs’ complete genetic profiles, researchers found a genetic tweak that can cause blue-eyed dogs. The study was published last week in the journal PLOS Genetics by Adam Boyko and Aaron Sams of Embark Veterinary, Inc.
Researchers tested the DNA of more than 6,000 dogs whose owners had purchased DNA test kits to confirm or identify their dogs’ breeds and explore their potential risk of health conditions. The size of the study was possible because of the scientists’ access to vast data thanks to the test kits purchased and owners’ willingness to participate in an online survey where they could also share photos of their dogs.
Researchers found that a genetic change, or mutation, near a gene known as ALX4 on canine chromosome 18 is strongly associated with blue eyes in Siberian huskies.
Genes are all about cause and effect, like dominoes toppling over, says geneticist Kristopher Irizarry of the College of Veterinary Medicine at Western University of Health Sciences. “One gene will turn on another, or turn off another, or turn off 10 and turn on others,” he says. “Genetics is this very complex spider web of pushes and pulls—and order and time are critical.”
Paths to Baby Blue
Blue eyes are found in a number of dog breeds, including Old English sheepdogs, border collies, and Welsh and Pembroke corgis, Irizarrypoints out. But for these breeds the blue-eyed trait is inherited as a recessive trait, meaning that two mutated copies of the gene are required for the blue eyes to occur.
In humans, he says, blue eyes are caused by a genetic variation between a pair of genes called HERC2 and OCA2 in the human genome.
According to Irizarry, the mutation of the ALX4 gene in Siberian huskies seems to result in decreased pigment production in the eye. The lack of pigment causes the eye to appear blue.
“There’s no blue pigment. It’s about the way the light enters and exits the eye, creating the appearance of blue, the same way the sky looks blue but outer space is not blue,” says Irizarry.
The type of mutation found in the study—in this case, the duplication of a snippet of genetic information—is also how tri-colored Australian shepherds sometimes end up with blue eyes, a phenomenon unexplained before this study, says one of its authors,Embark Veterinary, Inc. senior scientist Aaron Sams.
Doggie DNA Bonanza
One of the most remarkable aspects of this study, Irizarry says, is that the scientists had so much data to work with. In a study he’s working on to explore neurological disease in German shepherds, collecting DNA from even 40 dogs was a challenge, requiring a lot of driving and carrying around equipment to collect each DNA sample.
“With 6,000 people getting DNA samples from their dogs and mailing them to a centralized location and then filling out a website form detailing all the traits of their dog—that’s a game-changer for how genetics is being done in the 21st century,” he says.
Bridgett vonHoldt, a biologist at Princeton University, agrees. “It’s great that a private company can get owner approval to use samples for two purposes, the intended commercial product and additional research,” she said via email.
An added benefit is that pet owners get to contribute to science, says Irizarry. It’s also a big step for citizen science. “Each owner’s pet not only inspired the work, but actually contributed DNA to the study. This is very cool.”