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- Thank you to KiwiCo for supporting PBS. Maybe your thumb bends like this, or maybe it's straight. Do your ear lobes attach or do they flap freely in the wind? Some of us can roll our tongues while others can't. You might have learned in school that these traits are genetic, caused by one gene, the dominant and recessive versions duking it out in one of those games of genetic four-square. But there's just one problem: that isn't true. Most of our traits are nowhere near as simple as biology class would lead you to believe.
The real story of your genes is much weirder. It turns out you can have blue eyes even if both your parents have brown and red heads actually aren't going extinct. And whether you love or hate cilantro, that might not be all about your genes after all. Welcome to the genetics casino, where the odds are very rarely what they seem. Hey, smart people, Joe here. Let's go back to the 19th century Austrian empire. You might remember this guy, Gregor Mendel, a monk, gardner, and the father of modern genetics.
Well, people in Mendel's time knew that we inherited traits from our parents, but they had no idea how, and Mendel wanted to figure this out. So he of course, spent a decade or so cross-breeding more than 28,000 pea plants. That's one way to spend your time, I guess. But as he tallied the traits in his peas, he started to notice some patterns in the numbers. Seed shape, seed color, pod shape, pod color, plant height, flower color, and flower position. Each trait seemed to come in two different flavors or forms.
Mendel noticed that when he crossed plants with these different flavors of these traits, he could predict the ratio of the traits in later generations. Mendel didn't know about DNA or genes. These wouldn't be discovered until decades later, but he came up with the idea that the traits he was investigating were controlled by something physical passed from parents to their offspring, what he called hereditary factors. Take the color of the pea flower, Mendel bread line of plants that always produced purple flowers and one that always produced white flowers, as long
as they were only crossed with themselves--AKA pure bred peas. When he crossed a pure-bred purple pea with the pure bred white pea, the resulting offspring all had purple flowers. Purple dominated white. But when he bred those purple flour plants together, something strange happened. About one in four plants had white flowers. Despite two purple parents, the white trait was hiding in there all along. Now, a couple decades after Mendel's death, someone invented the Punnett Square to help us visualize all this. The inventor, Reginald C Punnett.
It's quite a coincidence. Let's use capital P for purple and lowercase P for white. In our first generation of flowers, we have pure-bred purple and pure bred-white and each plant carrying two copies of its factor. Now because purple is the dominant trait that beats out white, any pair that contains a capital P will have purple flowers, but cross two purple flowers that are secretly carrying the white trait, and you get the little PP combo in a quarter of your offspring, where the recessive white trait is finally visible. The Punnett square is a simple way to see one of Mendel's biggest revolutionary discoveries that after counting and recording all of those thousands of Ps, the only way
that his numbers made sense is if each individual P carries two copies of a trait, or as we now call them today, genes. This is true for humans too. We all have two different versions of each gene because we inherit one set of chromosomes from each of our parents, each carrying slightly different versions of a gene called alleles. Mendel did his work almost a hundred years before the double helix was discovered, but today we know that purple P flowers get their color from a pigment called anthocyanin, which is made through a series
of chemical steps, kind of like a factory assembly line. Now, the big P version of the flower color gene makes an enzyme that is key to this pigment assembly line. Pigment gets made and you have a purple flower, the little pea version. It's a broken enzyme, no pigment, white flower. As long as you inherit one working copy, you get purple. Only when both are broken do you get white Mendel's. Meticulous experiments prove that his plants were inheriting a pair of traits, one from each parent. And the unique combination of dominant and recessive traits determines what the offspring looks like.
It's a nice and tidy way to think about how traits get passed on, right? One gene, two possible versions, and one always wins. Brown eyes: dominant. Blue eyes: recessive. Free ear lobes: dominant. Attached ear lobes: recessive. Can roll your tongue: cool trick and dominant too. And so it was the simplicity of Mendel's Peas and Reginald Punnett Square lived happily ever after in every intro biology class, forever forevermore. Except no, it turns out Mendel's peas worked out into that nice, neat, dominant recessive pattern with one gene and two alleles because Mendel got lucky.
The traits that he chose to look at, they turn out to be some of the few that actually work this way. And almost no human traits are this simple. Let's start with a trait. We all know about:eye color. If brown is the dominant allele and blue's, the recessive allele, then you might think if two blue-eyed parents have a brown-eyed baby, somebody's got some splaining to do, you are not the father. But here's what's actually happening. You see those thread like textures in your iris? These are collagen fibers and they are naturally white.
They have no pigment. If melanin gets deposited in those fibers, then your eyes look brown. But without melanin, the fibers stay white, but they scatter light in a way that makes them look blue. Similar to how a bird's feather isn't actually blue, it just looks that way because how it bends light. Turns out whether an eye has color depends on more than one gene. You see, there's one gene that helps make the pigment and a second controls how much pigment the first one makes. Eye color is what we call a polygenic trait.
It's determined by many genes. In fact, one recent study counted 169 genes that are somehow related to how melanin is produced in your body. And at least 10 of them are involved in eye color. It doesn't exactly fit on one of those neat little squares. This is a pretty common situation actually. We're seeing the effect of one mutation depends on the presence of another mutation. This is called epistasis. Imagine that there's a gene that can cause either red hair or blonde hair. If there's also a different gene for baldness, then we never see the effect of the hair color gene in the first place. With 160
or so genes contributing to melanin production in the body. There's some other common myths out there about skin and hair color too. For example, if red hair were a simple recessive trait, we would absolutely expect it to die out as it was continually overpowered by more dominant hair color alleles. But more than 120 genes contribute to hair color, and none of them work quite as simply as that. And much like how red and white paint mix to make pink incomplete dominance is where both genes give a little. And the result is a blend between the traits. There's even cases where two versions of a gene might be present and instead of dominating or mixing, they're both fully expressed side by side or co-dominance.
I think we're ready to debunk some other genetics examples from your biology class. Okay, stick out your tongue between 65 and 80% of you can do this. You might have heard this is a classic genetic trait. Rolling is dominant and non-rolling is recessive. Except when scientists research this, which they actually did, it turns out that 20% of kids who couldn't roll their tongue at age six could do it by the time they were 12. Even better identical twins who share all of their DNA sometimes differ in tongue rolling ability. If it were controlled by a single gene, that shouldn't be possible. So genetics might play a role, but our genes aren't our destiny.
Tongue rolling is probably something you can learn just as much as something you're born with. What about ear lobes? You can probably guess where this is going. But thanks to a 2021 study of over 74,000 people, we found that ear lobe attachment style is influenced by at least 49 different genetic regions. They control things like skin characteristics, muscle and bone development, and how our bodies grow. That's why ear lobes come on a spectrum: fully attached, fully free, semi free, hang low, wobble to and everything in between. The same exact story goes for cheek dimples and cleft chins.
None of these fit into our neat little dominant recessive squares. And now a polarizing one. Chances are you've heard that some people hate the taste of cilantro because of their genetics. Blaming your DNA for your food choices is a convenient excuse at taco night, maybe. But say it with me now. It's not that simple. We have found genetic changes that are more likely to show up in people who think cilantro tastes like soap. One single letter change in this gene called OR6A2 can make someone more sensitive to these certain chemical compounds found in both
cilantro and soap products. But here's the twist: genetics only determines whether you can taste certain flavor chemicals. What it actually tastes like to you, though: that is way more subjective. People who grow up eating cilantro in a culture where it's a really huge part of the food landscape often learn to like it, even if they have that DNA change for sensitive flavor. Whether food tastes good is about a lot more than genetics. I mean, how else can you explain durian?
I'm fine. In fact, that's a pretty good equation for humans in general. You are the sum of your genes, your mutations, but also your culture and your experience and your education. Turns out there are very few human traits that are truly controlled by one gene with two simple alleles. And pretty much none of the ones we got taught in biology class make the list. But there are a few that do work that way. For example, earwax. Yours is either wet and sticky or dry. And this trait is actually one of those rare one gene two allele situations. Since I know you're wondering, I have sticky earwax.
Put it on my Wikipedia page. Red green colorblindness is a simple example because it's carried on a sex chromosome. So the math is actually even simpler. Genes that are carried on the sex chromosomes, we're talking about the X and Y that we inherit from our parents that determines our biological sex. They are particularly powerful because it means that in males, there's only one copy of some genes because they only have one X chromosome. So if they get a certain allele of a certain gene, there's no chance for it to be out competed by another allele.
Say a male carries the red green colorblind allele on the X chromosome he got from mom. Well, there's no second copy to make up for it, and he can't see this. This is also why orange cats are more likely to be male. The gene that determines orange fur color is on the X chromosome. So males who carry a copy of that recessive trait show it everywhere because there's no second copy to cover it like there is in female cats. Aside from these very few simple traits and maybe a handful more, it turns out that everything that makes us is pretty complex and messy. And that colorful, unpredictable chaos
is actually an excellent description of humanity, I think. And this makes sense, right? Because we are complicated organisms and even the simplest, most mundane things about us are still the result of incredibly biologically complex processes and systems interacting with each other. You know, the same way that just about every food that you love requires more than one ingredient cooked together, just the right way. You and I are too complex to fit into nice little boxes too. Stay curious. Thanks for watching. Leave us a comment and let us know what kind of earwax you have. And before you go, we want to hear your feedback.
Every year, PBS Digital Studios conducts a huge audience survey, which we actually use to help understand what you enjoy on YouTube and what you would want to see us make more of other than like a hundred more Be Smart episodes. You also get to vote on new show ideas. So it'd be great if Be Smart fans went out and represented in the polls. There's a link down in description. Thank you in advance. And thank you to KiwiCo for supporting PBS. KiwiCo is a monthly subscription that delivers hands-on science and engineering projects for kids of every age and interest. Each crate is like a mini lab in a box filled with everything kids need to build and create projects and learn through play. For young dinosaur lovers, age six to nine,
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including these fine folks at our top tier. As always, you can join that fine club by clicking the link down in description, and I'll see you and all of your beautiful mutations in the next video. And redheads are. .and whether the reason you hate, ugh, lover hate cilantro.that's gross.
