Hypothetically, if all the dogs in the world could somehow run wild, how many generations would it take before they all looked the same?

-A high school student from California

October 23, 2008

This is a very complex question. It might even make a good college-level genetics exam problem! The answer is that domestic dogs would probably never all look the same.

We would, for sure, quickly lose all the specific breeds that have been created in the last 150 years or so. But all dogs would not end up looking like Benji.

There would still be black dogs and brown dogs, big dogs and small dogs. To understand why, we need to dig a bit deeper into genes and how they work.

Genes and Their Different Versions

As you probably know, traits come from genes and the environment. For our discussion here, we'll ignore the environment and just concentrate on the genes.

Each trait can come from one or many different genes. And each gene can come in different versions.

What makes a Great Dane different from a Chihuahua is the combination of different gene versions each has. They share the same set of genes.

Dogs (and people) also have two different copies of each of their genes -- one from mom and one from dad. Each copy can be a different version too. All the different kinds of dogs out there are the result of different combinations of different versions of the same genes.

Here we come to an important point. Different gene versions sometimes do not mix. Instead one gene version can trump another. This is why dogs can sometimes look more like one parent than the other. Or one grandparent.

For example, a dog with a black version and a brown version of one particular coat color gene (TYRP1) has a black coat. The black version trumps (or is dominant) over the brown. Or you could say that the brown is recessive. The dog is not a mix of black and brown.

Here we come to another important point. In a large population of animals, amounts of gene versions do not change. Only combinations of those versions will change. Unless people (or nature) starts picking certain ones to breed.

What this means is that even when all the dogs of the world have interbred, all of the original gene versions will still be there. In other words, the combinations will still be possible for big brown dogs, little black ones, etc.

This also means that sometimes hidden traits will suddenly appear in new pups -- a pup may look very different from either mom or dad. So no matter how much mixing there is, there will still be lots of different kinds of dogs in the world.

Let's look at some real (but simplified) examples of coat color genes to see how this all works.

Coat Color in an Example Breeding

Researchers have found seven genes (so far) that affect the color and pattern of a dog's coat. This is pretty complicated so let's first only consider the gene we mentioned earlier, TYRP1.

Again, this gene comes in two main versions--black and brown. The black version is dominant over brown. (We are ignoring stripes, spots, etc.)

To make things easier, scientists call the black version, B, and the brown version, b. So there are three genetic possibilities here. A dog can be BB, Bb, or bb. Remember, one gene copy from mom and one from dad.

Because black is dominant over brown, BB and Bb will be black and bb will be brown. Now imagine that breed 1 is black, BB. And breed 2 is brown, bb. When these two breeds mate, the first generation will all be Bb ("genotype") or black ("phenotype"). The next generation will be very different though.

The next generation will be black and brown. Genetically, they will be BB, Bb, and bb.

As long as nothing selects for brown or black dogs, then this ratio will stay the same. In other words, if people aren't picking just the brown dogs or brown dogs aren't more likely to get sick, then for every 3 black dogs, there will be a brown dog.

Even if something did select for just black dogs, there would be an occasional brown one born. Because if two black Bb dogs have pups, odds are some will be brown.

Adding Another Gene

Now let's add another gene that comes in two versions. This gene is called MC1R. It has two versions which scientists call E and e. E is dominant to e. Here is where things get interesting.

The E version allows our previous gene (B and b) to show itself. So, when there is at least one E, the dog will be black or brown. But, when a dog has both copies as e, it could be red, yellow, or cream. But not brown or black!

So now we could have 4 genetic possibilities in our simplified example. B_E_ dogs will be black. B_ee dogs will be red, yellow, or cream depending on other genes. bbE_ dogs will be brown. And, finally, bbee dogs will be again be red, yellow, or cream. (The _ simply means because there is already one dominant gene, the other doesn't matter for the color of the dog.)

Now what would happen if we have dogs of all types that breed with each other? Well, it depends on the starting numbers of each gene version in each color. But at the end, there would still be all the colors in the population.

Even when we expand to 7 or more genes, the same ideas hold. The dogs will not all be the same color. Different combinations of genes will stay in the population so different colors will stay too. And this holds for all the other traits that dogs have too.

So there won't be one uniform looking breed of dog. There will still be a wide variety of very different looking dogs.

What about Selection?

All dogs came from wolves around 10-15,000 years ago. But in our story here, dogs do not revert back to wolves. This is because there is no selection in our story.

Selection is one of the main ways that amounts of gene versions might change over time. Let's briefly consider an example.

What if there were a group of these dogs living in an area that was very sooty or dark most of the day? In each generation, the brown dogs (bb) would be "selected against." In population genetics, this is known as having a lower "fitness."

This would remove some of the brown (b) versions from the population. Making the B (black) version more abundant.

In this case, black dogs would slowly become more common. However, because dogs can be Bb (black), we will occasionally still see brown dogs. Even when selection is playing a role!

So, there could be a difference in fitness for each trait that controls part of dog appearance. If this were the case, we would see a faster move towards dogs that look more similar. But again, there would still be many different versions that hang around forever.

Wrapping It All Up

We've seen that there are a large number of genes that control dog color. Even more control size, shape, etc. On top of that, these genes come in different versions. The versions also interact with each other in different ways. It's very complex! Scientists aren't close to understanding it all, yet.

And we haven't even talked about how the environment might cause differences between dogs. Food, climate, numbers of brothers and sisters -- all of these may influence their appearance as well.

We now also know that with no selection, the amount of these different gene versions will never change. Just the combinations change.

What does it all mean? Again, we would quickly lose distinct breeds. However, because all the variation is still there, we will still have a large amount of different looking dogs.

By Jared Wenger, Stanford University

Even if dogs breed freely, there will always be lots of different kinds of dogs.

Dogs will not revert back to wolves.