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Go to Brilliant.org/SciShow to learn more. [♩INTRO]. You might have noticed that a lot of corgi mixes look exactly like… a corgi in a costume of a different dog breed.

The Internet certainly has. Go pull up some pictures. We'll wait.

No matter what, though, something about these pups always screams “corgi.” It's the stumpy legs that gives them away. Those legs aren't just a coincidence, though. They're actually the product of thousands of years of breeding.

Or… inbreeding. And those little legs come from a funky mutant gene that's been passed down the whole way. Every corgi carries genes for dwarfism.

And purebred corgis have been bred to make sure that trait gets passed on every time. But when corgis breed with other dogs, their pups often have those same little legs, because that gene is dominant so even if a dog has only one copy of the gene, short legs will win out. Except that's not the whole story.

In corgis, dwarfism isn't caused by any old gene. It's caused by a mutant gene called a retrogene. A retrogene forms when the usual system for building proteins doesn't go as planned.

In a normal case, a gene, made of DNA, is transcribed into a molecule called messenger RNA, or mRNA. The cell then interprets that code and uses it as an instruction manual to build a protein. Which usually works great!

Except, in rare cases, that mRNA code which is only supposed to act as a messenger gets converted back into DNA. And in even rarer cases, that converted DNA manages to slip back into the genome—in a different place. Usually that gene doesn't do anything; it just sits there.

But under special circumstances, that duplicated gene can even produce proteins. That's exactly what's happening with the short-legged dog imposters. Multiple genes can cause dwarfism in dogs, but in corgis, and corgi mixes, dwarfism is caused by a retrogene.

Specifically, there are multiple retrogenes derived from a gene called FGF4. It codes for a type of protein called a growth factor that tells the body's cells how to grow. And that's a normal gene that all dogs have on chromosome 18.

But it's become a retrogene and copied itself back into the genome -- more than once. There's one of these retrogenes on chromosome 12, and one on chromosome 18. Either one is enough to cause dwarfism in dogs.

But some really stumpy breeds -- like corgis and dachshunds -- have both. Scientists think that the expression of these extra growth factors may alter the dogs' development. In fact, similar genetic changes are linked with dwarfism in humans.

In short-legged dogs, the extra FGF4 tells limbs to stop growing early, so the ends of their bones harden, and you end up with a regular-sized dog with stubby legs. And many breeds, from Pekingese to basset hounds, have short legs for similar reasons! That tells us two things.

One, this mutation appeared a really long time ago, back in the early days of domesticating dogs. And two, humans throughout history have found this trait so attractive that they bred these waddly dogs into existence all around the world. The cuteness is not without its downsides, though.

The same retrogene that gives dogs their adorable waddle is also linked with spinal disease that can cause pain and neurological damage, which scientists are still trying to understand. So next time you see one of those dogs in a corgi-shaped package, remember it's because humans have a soft spot for those little, furry mutants and probably have ever since our long friendship with dogs began. Now that you've had your dose of cute internet dogs, if you're up for a challenge, you could head over to Brilliant.org.

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