Thoughts on Dog Breeds

What makes dogs so different?

Every morning I take my two dogs for a walk. Cody is a lanky foxhound with long goofy ears and Max is a Jack Russell Terrier with short legs, stocky body, and perky ears. They couldn’t look or act more differently. Cody follows his nose and only long years of training, and his perception that I am the pack leader, makes him finally relinquish that delicious smell and follow along. Max, on the other hand, is completely fixed on me, never more than 20 feet away and wired for action. He is a little Napoleon who will take on all comers. These two purebred dogs are great examples of their breed groups – scent hounds and small terriers. They are testimony to the power of DNA to determine both the physical traits and behaviors that make dogs the perfect reflection of our needs.

The dog is the only large carnivore that was successfully domesticated and shows the greatest diversity of any domestic animal. The dog is a great laboratory to study how man has manipulated the natural world to our advantage. How was such enormous diversity achieved from the basic wolf template? Size, body type, head shape, ear shape, leg length, and, most interesting to me, behaviors had to be manipulated to create the great dogs of today. These traits and others are incredibly different between dog breeds.

What about DNA and Mixed-breed Dogs?

Several laboratories now offer a DNA –based test to determine the breed mixture in mixed breed dogs. First, it is important to point out that these tests are not designed to prove that your purebred dog is 100% whatever. Most of our modern dog breeds arose in the last 150 years and have too many genetic similarities to be distinguished with 100% accuracy. That number is important; these tests can in many cases show a strong likelihood what pure breed a dog is but can rarely do so with 100% accuracy. Usually these questions arise because a dog purchased as “pure” looks a bit “off” and there is a financial/legal motive behind the question. Without the ability to offer 100% accuracy, none of the laboratories will suggest using the tests for anything other than estimating what pure breeds went into your most excellent mixed breed dog.

How Do These Tests Work?

The testing are is based on a type of genetic marker called a SNP; that stands for Single Nucleotide Polymorphism. If you take a bunch of dogs and sequence their DNA and then compare it, you will find sections that are exactly the same and sections that have some differences.  Scientists have classified the nature of these differences and they call stretches of DNA that differ by only one DNA base (nucleotide) as SNPs. DNA is made up of a long succession of only 4 bases, abbreviated as A, T, C, G. Imagine the same DNA stretch from 2 dogs:

Dog 1    :  TGCTGAACTTGCTTAG
Dog 2    :  TGCTGAGCTTGCTTAG

You can see that the 2 dogs show a difference indicated by the bases shown in BOLD; in science lingo that is a ”polymorphism”.  Hence the name: a SNP is a single base difference between 2 individuals in the same DNA stretch.

Now let’s says that hundreds, even thousands of dogs have had their DNA characterized and compared. Lots of these SNPs have been identified. But what do they mean? Do they represent DNA from the genes that make hounds different from terriers? Possibly, or possibly not. But they don’t have to be – for breed tests the differences must only have some predictive value in distinguishing hounds from terriers. In other words- rather than starting with genes that we know are different between dog breeds,  scientists have used the power of math and statistics to come at the question from the other side. Now that we have seen DNA differences we ask “what do they mean”? Using the power of computers, scientists have examined thousands of SNPs in hundreds, even thousands of purebred dogs and seen that certain ones are more likely to appear in one breed than another. Moreover, the SNP patterns group dog breeds as one might expect based on the history of their origins.

Are Some Breed Testing Labs Better?

The quality of these tests is determined by the number of SNPs tested and the population of dogs that formed the basis for comparison.  Imagine a lab has a database of 50 representative breeds, 10 dogs per breed (500 dogs total). Compare that with the same number of SNPs from 125 breeds, 40 dogs per breed. (6000 dogs total). You would expect more accurate estimates from the larger and more diverse population. If the breed that makes up a big fraction of your dog’s lineage wasn’t part of the 50 breeds, then the estimate will be off. The “quality” of the population is also important. How are dogs identified as “pure”? By their owners? By registry records? Having a larger database helps to correct for a few errors in breed identification. Testing more breeds, and more dogs per breed, enhances the accuracy of the test. There is lots of information for the consumer to pursue: an article for veterinarians, an online article for dog owners.

So this SNP analysis has been done not only by the laboratories offering breed testing but also by consortia of scientists worldwide to group pure breeds and to trace how and where the domestic dog arose from their wolf ancestors.

This diagram shows the groupings of dog breeds relative to the each other and to their wild ancestor, the wolf.