What makes a dog breed a dog breed? Let’s answer that question using one of the most popular breeds today as an example: the Corgi. For starters, the signature bobtail in the Pembroke Welsh Corgis comes from a mutation in the T-box gene. Without it, the Corgi everyone knows and loves would look totally different.
Another mutation, one in the FGF4 gene, gives the Corgi those disproportionately short, sturdy legs a condition known as chondrodysplasia. It’s the same mutation that causes short legs in the Dachshund and the Basset Hound.
To put it simply, dog breeds are defined by specific, recognizable physical and behavioral traits. Breed-specific traits are coded for by DNA variants that appear at a high frequency in a dog breed. Many breed-specific traits are dictated by single nucleotide polymorphisms or SNPs. These usually segregate in two forms or alleles such that the frequency of one can be inferred from the other. Traits driven by structural variants may have more allelic diversity. That’s why when two Corgis breed, the litter is guaranteed to be Corgi puppies. What if a long-legged Pembroke Welsh Corgi shows up in the litter? Variations in offspring like this are possible because they’re driven by genetic changes, but the resulting puppy won’t meet the breed standard.
It’s the DNA that defines the breed. And what we report on is the DNA.
What if you have a mixed-breed dog, one that isn’t clearly the product of two purebred dogs? What explains a 40-pound black Labrador-ish mutt with Corgi and Yorkshire Terrier grandparents?
If you were to get results like this back after testing your dog’s DNA with Embark, your first inclination might be to assume we contaminated the sample somehow. A sample contaminated with another dog’s DNA will fail our quality control: if we get more than two different copies for a segment of your dog’s genome (called a haplotype), barring the unlikely event that the dog is aneuploid (with an abnormal copy number of any given chromosome), we know that more than one dog’s DNA is in the sample and will issue you a re-swab immediately.
Genetics is a game of chance.
First, we’ll address our hypothetical dog’s Corgi maternal grandparent.
The hypothetical grandparent carries characteristic Corgi DNA variants like the aforementioned FGF4 mutation for chondrodysplasia. Our Embark dog has a 50% chance of inheriting this allele from the mother. This also means he has a 50% chance of not inheriting this Corgi allele from his mom. If your dog does inherit the Corgi allele, he may have intermediately short legs. If he happens to inherit a matching allele from his dad, he’ll get fully short legs.
Let’s make it a little more complicated and address a trait affected by many genes by examining our hypothetical dog’s Yorkie grandparent.
Among other things, Yorkies are prized for their lap-friendly size. An allele of the IGF1 gene is known to cause small body size in dogs, including the Yorkie. Going through the same calculations as above, your dog has a 50% chance of inheriting this Yorkie allele. Even then, the effects of this allele will be modulated by the variants at other size-influencing genes identified in the dog, as well as those that haven’t. Depending on what your dog has inherited from both his mom and his dad’s side at those other loci, his size may not ultimately reflect the Yorkie size that the IGF1 allele confers.
This is all a game of chance. This is why mixed-breed dogs of similar breed composition can be completely different in body shape, size, and coloring. For example, all three of the Embark dogs below are between 18 and 23% Chow Chow.
The vast majority of our DNA doesn’t code for genes, but is highly informative of ancestry.
Our hypothetical Embark dog didn’t inherit a Yorkie size allele. He also didn’t inherit a Yorkie black-and-tan agouti coat color allele. Or maybe he did—coat color in dogs is influenced by more than seven genes, some of which actually mask the effects of others. Maybe he didn’t get the Yorkie’s penchant-for-barking-allele, either. So what’s that 25% Yorkie DNA doing? Don’t forget that only 2% of our DNA codes for genes, which make the proteins that eventually lead to phenotype. The rest of it is largely regulatory in nature.
Of the 230,000+ genetic markers that we use in our testing, roughly 10% of them fall in the coding sequence. The rest is in highly conserved DNA that doesn’t code for genes. This is incredibly useful for high-resolution breed identity, but may not directly impact what your dog looks and acts like. It’s the DNA that defines the breed, and what Embark reports on is the DNA.
DNA makes the dog, but so do you.
It’s important to go into testing your dog’s DNA for breed mix with an open mind. Some preconceived notions about your dog’s breed are bound to go out the window when you have an as high a resolution of a genotyping platform as Embark’s. Ultimately, does your dog’s breed composition change who your dog is to you? No: You were the one who taught Fido to sit, to go nuts for peanut butter, to love playing tug of war. It’s the classic case of nature versus nurture. Perhaps Fido has Labrador in him, and that’s why he likes to swim. Or maybe it’s because you threw him in the lake when it was hot out one day, and he figured out that he loves the water.
The value of the DNA test is not only to figure out who your dog is. It is also the ability to anticipate breed-specific conditions and diseases. For example, Staffordshire Bull Terriers are known to experience a condition known as Staffy Cramp, a neuromuscular disorder that arises from a mutation in the L2HGDH gene. This breed may be found in many dogs identified as pit bull mixes found in shelters. But studies suggest that the reliability of visual identification of dog breed is low, especially with Staffy-type dogs. In cases like these, owners are more prepared for their dog’s health when they have the understanding a DNA test brings.
We choose our dogs unscientifically. Some of us have a few criteria of what we want: breed, size, activity level, and etc. But ultimately, our decision is based more on emotion than rationale. And with that decision comes the responsibility for this dog for the rest of their life. It is invaluable to know what kind of breed-specific disease and trait alleles are coming with your new pet.