A study of purebred and mixed-breed dogs shows that distinctive physical traits are not highly correlated to inheritable behavior traits.
Dog genetics suggest that behavior is more than just breed
Dogs and people have lived as companions for thousands of years. However, it has only been within the last 150 years or so that people have selectively bred dogs for specific combinations of physical traits that we now associate with dog breeds. Many people define individual breeds as much by their behavior characteristics as their by physical ones. But a genetic study by investigators at UMass Chan Medical School and the Broad Institute of MIT and Harvard scientifically challenges these notions about dog breed stereotypes and personality types.
The study of 2,155 dog genomes and 18,385 owner surveys, published in Science, identifies 11 unique genetic regions (or loci) in the dog genome that are strongly associated with behavior. None of these regions were specifically associated with any particular breed, suggesting that these personality traits predate modern canine breeding by humans.
Canine behavioral disorders are often proposed as a natural model for human neuropsychiatric disorders. Compulsive disorders, for instance, are often observed to manifest similarly in both humans and dogs. For this reason, canine genetic studies have the potential to identify loci in the dog genome that could lead to new insights in humans. Karlsson and colleagues show here that large-scale genetic studies of dogs can indeed yield genetic loci associated with behavioral traits.
"While genetics plays a role in the personality of any individual dog, the specific dog breed is not a good predictor of those traits," said study senior author Elinor Karlsson, who is an associate professor of molecular medicine at UMass Chan Medical School and director of the Vertebrate Genomics Group at Broad. "A dog’s personality and behavior are shaped by many genes as well as their life experiences. This makes them difficult traits to select for through breeding.
"For the most part, pure breeds are only subtly different from other dogs," Karlsson continued. "Although friendliness is the trait we commonly associate with golden retrievers, what we found is that the defining criteria of a golden retriever – what makes a golden retriever a golden retriever – are its physical characteristics, the shape of its ears, the color and quality of its fur, its size; not whether it is friendly. A golden retriever is only marginally more likely to be more friendly than a mixed-breed or another purebred dog, such as a Dachshund."
Untangling behaviors and breeds
While researchers peg dogs' emergence from wolves to about 10,000 to 15,000 years ago, humans did not begin intentionally breeding dogs until roughly 2,000 years ago, selecting them for work roles such as hunting, guarding, and herding. It was not until the Victorian era in the 1800s that humans began selecting dogs consistently for the physical and aesthetic traits that today we commonly associate with modern breeds.
Breeds are often credited with characteristics and temperaments (bold, affectionate, friendly, trainable) that correlate to their ancestral function. Likewise, a dog's breed ancestry is assumed to be predictive of temperament and behavior. DNA tests are even marketed as tools for dog owners to learn about a pet’s individual personality. However, there is a lack of genetic studies linking behavioral tendencies to ancestry or other genetic, heritable factors.
To explore the complicated relationship between modern dog breeds and behavioral characteristics, Karlsson; the study's lead author, graduate student Kathleen Morrill; and their colleagues applied a strategy called a genome-wide association study (GWAS) to connect areas of the canine genome with certain traits or characteristics. For their data, they turned to Darwin's Ark, an open-source database of owner-reported canine traits and behaviors.
Pet owners who participate in Darwin’s Ark provide scientists with saliva samples from their dogs. Researchers run whole genome sequencing on these samples to generate a robust genetic data set for investigation. Additionally, for this study, owners filled out 12 short surveys totaling 117 questions about their pet’s behaviors and physical traits, developed using standard definitions for reporting and rating canine behavioral traits, and physical and aesthetic standards established by the American Kennel Club.
All told, Karlsson and Morrill collected more than 2,000 canine genomes and 200,000 survey answers through Darwin’s Ark, representing 78 breeds.
"Given a large enough sample size, GWAS are a really powerful tool for learning about genetics," said Morrill. "We only get that size by looking at all dogs – not just purebred dogs but mixed-breed dogs too. We compare all these DNA sequences computationally, using complex algorithms, to identify areas of differences and commonalities that might be of interest."
Overall, Karlsson and Morrill found that behavioral characteristics were influenced by multiple factors, including environment and individual genetics, but that modern breed classification played a modest role in the outcome. By analyzing behavioral data across owner-reported breeds and genetically detected breed ancestries, the team found that breed's contribution to behavior was relatively small, only 9 percent. For certain behavioral traits, such as toy-directed motor patterns (a dog’s interest and interaction with toys), age was a better predictor of behavior: younger dogs were more likely to score higher in this category. For specific survey items, such as "lifts leg to urinate," a dog’s sex was the best predictor of behavior. Physical traits like coat color were more than five times more likely to be predicted by breed than behavioral traits.
Additionally, the investigators failed to find behaviors that were exclusive to any one breed. Even in Labrador retrievers, which had the lowest propensity for howling, 8 percent of owners reported their Labrador’s sometimes howl. Likewise, while 90 percent of greyhound owners reported that their dogs never bury their toys, three owners described greyhound dogs as frequent buriers.
The researchers also leveraged the genetic ancestry of highly mixed-breed dogs to test whether behavior is heritable in a breed-dependent manner. In some cases, heritable behavioral traits like biddability (a dog response to human direction) were somewhat more likely to correlate with breed, even if mixed a few generations back. In the case of purebred dogs, ancestry can make behavioral predictions somewhat more accurate. For less heritable, less breed differentiated traits, like agonistics threshold (which measures how easily a dog is provoked by frightening, uncomfortable, or annoying stimuli), breed was almost useless as a predictor of behavior.
By comparing dog genomes to identify genetic variations tracking along breed, as well as along individual physical and behavioral traits, Karlsson and Morrill identified 11 loci of the dog genome strongly associated with behavioral differences – none of which were specific for breed – and another 136 suggestively associated. The genetic differences between breeds such as golden retrievers, Chihuahuas, Labrador retrievers, German shepherd dogs, and others, primarily affected genes that control physical traits – far more than breed differences affected behavioral genes.
"The majority of behaviors that we think of as characteristics of specific modern dog breeds have most likely come about from thousands of years of evolution from wolf to wild canine to domesticated dog, and finally to modern breeds," said Karlsson. "These heritable traits predate our concept of modern dog breeds by thousands of years. Each breed inherited the genetic variation carried by those ancient dogs, but not always at exactly the same frequencies. Today, those differences show up as differences in personality and behavior seen in some, but not all, dogs from a breed."
Support for this study was provided by the National Institute of Mental Health, the National Cancer Institute, the National Human Genome Research Institute, the Office of the NIH Director, the National Institute of Aging, the National Science Foundation, the Broad Institute BroadIgnite and Broadnext10 programs, the Darwin’s Ark Foundation, the Food Allergy Science Initiative, the Working Dog Project and other sources.
Adapted from a press release issued by UMass Chan Medical School.
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