Doggone Legs: Dog Breeds with Short Legs Have an Extra Gene

An Extra Copy of the FGF4 Gene Has Been Around Longer than Most Breeds
July 29, 2009 How tall is that doggy in the window? If the answer is "not very," scientists can now tell us why. Wiener dogs, corgis, and basset hounds have short, stubby legs. It's not because they aren't eating their puppy chow. It's genetics. Scientists have recently found the piece of dog DNA that is responsible. Short-legged breeds have an extra copy of the gene "fibroblast growth factor 4," or FGF4. Long-legged breeds don't. And even though different short-legged breeds arose in different countries at different times, they have the same extra copy of FGF4. This suggests that the extra copy was made before the different breeds developed. This tells us why dogs have short legs. It's also a great example of how a short stretch of DNA can have long-term effects.
Short Legs and an Extra Gene

An extra copy of FGF4 causes some
dog breeds to have short legs.
There are over 350 breeds of domesticated dogs. In a new study, Heidi Parker and her colleagues took a look at the DNA of some of them. They compared the DNA of a lot of short-legged dogs to a lot of long-legged dogs. They found that short-legged dogs have an extra piece of DNA called an insertion. The extra DNA has a copy of the FGF4 gene. What this means is that instead of the normal two copies of FGF4 that most animals have, short-legged dogs have three copies. And since genes are the instructions for making proteins, an extra copy of the FGF4 gene may cause extra FGF4 protein to be made. FGF4 belongs to a group of proteins that are used as signals between cells. Basically, one kind of cell dumps FGF4 into the surrounding environment. This FGF4 then interacts with cells that have a different protein, the FGF receptor. The receiving cells then get the message to grow or move. The authors suggest that too much FGF4 over-activates the FGF receptor, and that causes short legs. But wait a minute; doesn't the G in FGF4 stand for GROWTH? It does, but growth is a tricky thing. One would think an extra copy of a growth factor would cause more growth, and more growth would mean longer legs, right? But a lot of a stimulus doesn't always mean a lot of the expected response. For example, turning up the volume on a stereo really loud will allow a teen to hear things better for awhile. Until he damages his hearing, that is. There's a healthy range for listening to music just like there's a healthy range for receiving FGF4. In humans, there's a mutation that causes an FGF receptor to go into overdrive (sort of like having an extra copy of the gene). This mutation leads to over 95% of one form of dwarfism. So it makes sense that FGF4 causes short legs in dog breeds. But how did scientists actually figure that out?

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Digging through Dog DNA
Sniffing out SNPs First, scientists looked at the DNA from a bunch of different dogs. But they didn't look at every single A, C, T, and G (nucleotide) in the DNA. That would have cost a fortune. Instead, they focused on some special spots. The vast majority of DNA in one dog is the same as the DNA in another. Fido's DNA looks pretty much the same as Lady's. But there are some differences, and they are important. For example, Fido may have CATTA at a certain position, whereas Lady may have CAGTA. These differences are called Single Nucleotide Polymorphisms (SNPs). Some of these SNPs affect something in Lady's or Fido's body while others do not. Whether they have an effect or not, scientists can use these SNPs as clues to track which parent a chunk of DNA came from. Dogs (and people) have two copies of DNA, one from mom and one from dad. Let's say Fido and Lady have a puppy called Duke, and scientists look at Duke's DNA. If Duke has CATTA and CAGTA, scientists can tell where each copy of DNA came from. CATTA came from Fido and CAGTA from Lady. SNPs allow scientists to track pieces of DNA. That's useful, but the scientists that figured out short-leggedness were not looking at dogs that were related to each other. They were looking at many dogs from many different family lines. So, how did they use the SNPs? When animals in the same species share a trait (like leg length), there are often a bunch of different SNPs associated with those traits. These SNPs are usually clustered close together. So the scientists figured that short-legged dogs all probably have the same letter at a particular SNP. And that letter is probably different in long-legged dogs. The scientists looked at a lot of different SNPs in a lot of different dogs. At the end of the day, they could say something like, "All short-legged dogs have an 'A' at SNP-52, but long-legged dogs have 'G'." Chasing down the right gene That was great information but it wasn't necessarily that informative on its own. SNPs don't always tell scientists what part of the DNA is actually causing something like short legs. SNPs just told scientists that whatever was causing it was close by. Understanding this requires an understanding of a concept called "recombination." Some people consider recombination a shuffling of DNA.

Where DNA is swapped in
recombination is random.
Think of getting DNA from parents like this. Everybody receives a book from mom and an almost identical copy of the same book from dad. As sperm or eggs are made, "recombination" occurs, which is like ripping out the last 100 pages of each book and attaching it to the other. The whole book is still there, but some pages came from mom's book and the rest from the dad's. Here's the important thing about recombination: pages that are close together will probably stay close together as they are passed through the generations. This is because where recombination happens is totally random. It might happen on page 16. Or it might happen on page 973, 242. So Lady's DNA from page 52 and page 53 will probably be inherited together by all of her ancestors. But something on page 1,000,000 will have less of a chance of being inherited with page 52. That's because it's much less likely for recombination to occur between pages 52 and 53 than 52 and 1,000,000. To separate pages 52 and 53, ripping could only occur right between pages 52 and 53. But to separate pages 52 and 1,000,000, ripping could occur at page 54 or page 55 or page 56 . . . and so on to 1,000,000. So, DNA sequences that are close together (like those on pages 52 and 53) are likely to be inherited together. If the DNA that is causing short legs is on page 53, odds are that it was inherited along with page 52 by all short-legged dogs. To summarize, scientists found that all short-legged dogs had a certain SNP. This told them that the gene causing short legs was nearby because the SNP and the gene were probably inherited together. While the SNP narrowed down where to look, it didn't tell them exactly what was causing short legs. But once scientists were close, they started looking at all the nucleotides in the DNA nearby to see if any changes in DNA made sense. For instance, perhaps a DNA change caused a protein to change, and that may have been the cause. In the case of the short-legged dogs, it was much more interesting. They found an insertion of FGF4 that was present in breeds with short legs but not breeds with long legs. This means that FGF4 was copied and inserted back into the dog DNA in a different spot. It's like taking page 795, where the original FGF4 was, copying it, and then sticking the copy into the book at page 53. From then on, FGF4 would almost always be inherited with the SNP on page 52. There it is. Geneticists used SNPs and the principle of recombination to hunt down FGF4. Figuring out that an extra copy of FGF4 causes short legs in dogs wasn't easy. Scientists had to use all their tools to find it. Andrew Hellman
Small differences in
DNA help geneticists
track down
important genes.
An extra copy of a gene
means extra protein.