Having Six Fingers Is a Dominant Trait

[INTRO ♪] 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. If you’d like to help us out, and earn some extra neat perks along the way, check out patreon.com/scishow. [OUTRO ♪]

100 thoughts on “Having Six Fingers Is a Dominant Trait”

  1. So an overabundance of Sonic Hedgehog causes weird mutations?

    Explains how the Sonic the Hedgehog movie came into being.

  2. This entire video feels like reading a layman's scientific paper after someone have used search&replace on some key word…

  3. I didn't realize "Shh" was an initialization of "Sonic hedgehog", I thought it was shushing us to stop laughing every time she said Sonic hedgehog. Then 2:48 happens and I got waaaay confused

  4. No offense to the other presenters, I prefer that skinny nerd kinda blond guy with glasses.
    I really miss new videos of him, and his faces and expressions. And his voice is also pleasant

  5. So.. if scientists find how which cells it takes for humans to grow a leg, people born without legs could grow one anyway?

  6. tuned out of this video to come back to "sonic the hedgehog" constantly being said… thought i landed on an april fools day video for a sec lmaoo

  7. TL;DR, if you have too much or too little Sonic HedgeHog DNA in the right or wrong places your fingers and limbs will develope differently.

  8. It's so hard to pay attention, take this seriously, and learn something when every other sentence includes the words "Sonic hedgehog". 😀

  9. I've had three polydactyl cats, one of them had extra digits on the back feet, and each of the three had thumbs on both front feet that looked like hands.
    By the time they were about three or four months old they were using their front feet like hand and picking up things.
    One with very large "hands" would sit on a cat tree and catch small balls thrown to him with one "hand".

  10. So if you have too much sonic hedgehog you get an extra finger, and if you have little to no sonic hedgehog you have less fingers. I’m glad I had just the right amount of sonic hedgehog.

  11. The person who named Shh has managed to get many smart and professional people to repeat the name of a fictional video game character over and over again in the most professional of settings for the most professional of reasons.

  12. Huh. My grandmother taught school in a very isolated community in the Ozarks back in the 1930's, and she told me once that there was an entire family of six-fingered individuals living there.

  13. a question that has bothered me since i started learning "genetics" in school, and i think there might not be an answer (?), but i really dont know
    what makes an allele dominant or recessive? (its kinda related to "how do stem cells know how and what to turn into", at least thematically, maybe biologically as well, but your guess is as good as mine)

  14. poly doesn't just mean many. it specifically means six, as well as more loosely many. hexa is the strict 6. poly an ealier loose six. so its sorta a pun

  15. Did you know that the company that created Sonic tried suing the biologists who named it that because they didn't want parents who had their children with the mutated version to be told that it was the "sonic hedgehog" gene that did it.

  16. I could not concentrate on what she was saying because I kept getting distracted by "sonic hedgehog" being a scientific term. XD

  17. There were a couple moments where I thought she was going to burst into laughter over the “sonic hedgehog” term…

  18. So interesting! Thanks. I only wish you had included some pictures of people with extra fingers. Also, I just had this idea. Since John Green is a bit of a self-taught expert on conjoined twins, I think it would be very cool if you had him guest host an episode on that topic some day.

  19. Speaking of slightly weird things, my son had three upper center front teeth when he was a toddler – two of them were fused together. When he was a little less than two years old, the fused area between them decayed and he had a hole right through the middle of the two fused teeth. He had to have it extracted, and therefore only had one center front tooth until he was about seven years old. That was pretty odd.

  20. My sister had two thumbs on one hand, now I know why… disappointed, thought she was a mutant with super powers! She could count to 11 before me…

  21. Scientists had no chill damn. "Sir, you're dying from a crippling never before seen disease. I just finished Links Awakening, so I'm calling it Link's Disease"

  22. Gotta love that gene's name. Floating around at the speed of sound, got places to go, gotta follow that rainbow. =P Doubly so where its not supposed to go. That aside, research into this is incredibly important, if things go wahoonie-shaped and we need to develop dna-repair treatments.

  23. You forgot to mention that there is a protein that suppresses sonic hedgehog expression called robotnikinin.

  24. I now know how my polydactl cat was put together, I can blame sonic the hedgehog. I have christen my cat with the sir name Deathclaw, which I think would be a better video game reference for a gene that helps bring about fingers and what not.

  25. I have a manager at work with an extra thumb. Oddly enough, her son does not have an extra thumb, and neither does her brother, but her niece (the brother's daughter) DOES have an extra thumb. Maybe the niece happened to have a random mutation that matches her aunt, or maybe she got it from her mom, and it's a coincidence that both sides of her family have extra thumbs.
    Also, for some reason, lots of people don't notice or realize that she has an extra thumb. Though it's more obvious when she paints the extra nail. Her thumb is kinda branched though, so I guess it's not so much two thumbs as a thumb with two tips.
    Also, there's been studies, and people of European and/or Asian descent (like my manager) are more likely to have extra thumbs, while people of African descent are more likely to have extra pinkies. No word on Amerindians though.

  26. I actually know someone who was born with 12 fingers and twelve toes but she amputated her extra fingers but still has six toes on each foot…

  27. Thanks for yet another excellent video. Can I ask for a possible follow up on this subject?
    -As mentioned the number of digits is darn easy to change
    -There doesn't seem to be any great disadvantage to having extra digits, or in some cases fewer (eg: Hemingway cats, ostrich footed people)
    -Five digits isn't even the "primitive condition"; most early tetrapods had more
    …and yet where there isn't an obvious reason (eg: birds) five digits is pretty tightly constrained among tetrapods, especially mammals – why?

  28. So my grandfather was born an extra pinkie on each hand and I was born with an extra rib
    My father doesn’t have any extras
    Super weird it skipped a generation

  29. But what causes my girl kitty's paws? Her thumbs stick out like a polydactyl, and I thought she was, when she first climbed up my leg to adopt me. But, she doesn't have extra claws. lol It took me 8 years to get a picture of her thumbs.

  30. SciShow, I'm pretty sure that you have the expression of Sonic Hedgehog on hand development reversed. I was taught that the area where it's effect is strongest is where the thumb develops and where it's weaker you get fingers. That's why your pinky finger is codependant on your ring finger when you try and move them separately but you'll have no problems wiggling your thumb by itself.

  31. I have a polydactle snow white cat. He's a very big kitty and he has 2 extra toes on each of his front feet. This makes his huge paws look like mittens. He is quite the snuggler too. With all these traits we named him Olaf

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