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In the Florida Keys at Ernest Hemingway's old house there's a whole colony of cats with too many toes. The story goes that Hemingway was once gifted a six-toed cat, named Snow White or maybe Snowball—the details are kind of sketchy—by an old sea captain.

Now, that cat's six-toed descendants roam freely on the estate. This story is so widely circulated that cats with extra toes are often referred to as Hemingway cats. But these felines' fancy feet can also teach us something—about the genes that build our bodies, and how they're passed down across generations.

The scientific name for too many fingers or toes is polydactyly, which comes from a combination of two Greek words: poly, meaning many, and dactylos, which means digits. It's not just cats who can have too many digits. Polydactyly can affect a variety of vertebrates, like chickens, dogs, guinea pigs and mice.

And humans, like that guy who killed Inigo Montoya's father. Although it affects many species, it's still pretty rare—even though much of the time, having six fingers is actually controlled by a dominant allele. An allele, or version of a gene, is dominant when it only needs to be present in one copy in your cells for its associated traits to be discernible.

So do you have six fingers on your right hand? No? Okay, don't prepare to die I guess.

Just because a gene is dominant doesn't necessarily mean it's common. It's actually a lot more complicated than that. The development of complex, finger-having organisms like us starts with one cell: a zygote.

That cell begins to divide into more cells, and at various points in the development process, those cells turn into arms, legs, a heart, a brain, and everything else. This process is called cellular differentiation, and what the cells turn into is dictated by your genes. Scientists first characterized one of these important developmental genes back in the 1980s in studies of fruit fly development.

They noticed that a group of flies with a specific mutation turned out stumpy and covered in spiky protrusions. So naturally they named the gene hedgehog, because it made the flies look a little bit like hedgehogs. And… fly biologists are like that, I guess.

Fast forward to the 1990s, where researchers had found three versions of the hedgehog gene in mammals—one of them being sonic hedgehog. Yes, named after that Sonic. The sonic hedgehog gene codes for a type of protein known as a morphogen.

A morphogen is a kind of molecular signal that gets sent out to cells to tell them what to become. When a cell detects the presence of sonic hedgehog, a complex chain of molecular events is touched off, eventually resulting in changes in how that cell's genes are expressed. How a cell responds to sonic hedgehog's signal depends on just how much of the protein it detects.

Which can lead to complex patterns, like our fingers and toes. Our hands and feet start out as a lump of tissue called a limb bud. Sonic hedgehog is especially concentrated in an area on one side of that lump of cells, and there's less of it as you go to the other end.

An area of tissue that detects a lot of sonic hedgehog becomes a pinky; none at all, and you get a thumb. If there's too much of that signal in the wrong place, though, other fingers start to develop—and that's how you get polydactyly. This is how a polydactyly-causing mutation can be dominant:.

A bunch of genes influence sonic hedgehog signaling, and it only takes a single faulty copy of a gene to put some sonic hedgehog where it doesn't belong. Depending on how much sonic hedgehog is present and where, you can end up with different outcomes. If lots of sonic hedgehog is present on the thumb side of the hand, that can lead to the development of two thumbs.

Cats with extra thumbs: their only weakness, solved. No can opener is safe. Now, there are a handful of other forms of polydactyly, influenced by a handful of different genes, as well as a condition called syndactyly, where a lack of sonic hedgehog signal results in fused fingers and toes.

Development requires a ton of genes to work together, and a single mutation can be enough to nudge sonic hedgehog all out of whack. Which just goes to show that building a human—or a cat—is a pretty impressive feat of cell sculpting. Thanks for watching this episode of SciShow, and thanks to our lovely patrons who support everything we do.

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