Tongue Rolling and 5 Other Oversimplified Genetic Traits
Can you roll your tongue? If so, you’re part of the majority. Between 65 and 81 percent of people on Earth have this strange and seemingly arbitrary talent. But why can some do it while others can’t? The most common answer, the one often taught in elementary schools and museums, is that it’s all about genetics. The story goes that, if you inherited a dominant variation of the “tongue rolling gene” from one of your parents, you too will inherit this party trick. In other words, if you can’t do it, blame mom and dad.
But John H. McDonald, a professor in the University of Delaware department of biological sciences, calls B.S. “If that were true, you could never have two non-rolling parents that having a tongue-rolling kid,” he says. “Yet people have looked at families and find you do see that.”
According to McDonald, teachers and textbooks have been over-simplifying this story for decades. The genetic theory of tongue-rolling can be traced back to a 1940 study by a scientist called Alfred Sturtevant that was quickly debunked. “By the early 1950s, people knew pairs of twins where one could roll and one couldn’t,” McDonald says. “That pretty clearly tells you it’s not all genetic. Yet I ask even today my students ‘how many of you have been told that tongue rolling is a simple genetic characteristic?’ and most raise their hands.”
The truth is a bit more complicated. McDonald says that in some cases, the environment plays a part. It's “nature vs. nurture” in action—many people can break genetic bounds and teach themselves the sacred art of tongue rolling. In other cases, it could just come down to a developmental quirk, like your position in the womb, he says.
So why has this rumor persisted? “It would be really nice to have a biology experiment you can do just by looking around the room,” McDonald says. But spreading these kinds of inaccuracies can be really dangerous. “It is an embarrassment to the field of biology education that textbooks and lab manuals continue to perpetuate these myths,” he writes. “If students took it seriously, a large proportion of students would look at mom and dad and conclude that the mom was sleeping around and dad wasn’t really their dad.”
Tongue-rolling isn’t the only genetic trait we’ve oversimplified. Here, a few other examples McDonald says he's debunked.
The myth: Whether you put your left thumb on top or your right thumb on top when you clasp your hands is determined by a single gene.
The reality: Even identical twins have different preferences for how to clasp their hands, indicating that there isn’t a “left thumb on top” gene.
2. Eye color
The myth: Blue eyes are determined by a single recessive gene. A brown-eyed kid cannot have two blue-eyed parents.
The reality: “Eye color is determined by variation at several different genes and the interactions between them,” McDonald says. “This makes it possible for two blue-eyed parents to have brown-eyed children.”
3. Hair color
The myth: Red hair is determined by a single gene that yields to other colors. Two red-headed parents cannot have a non-red-haired kid.
The reality: There are many variations in the gene that controls red hair pigment, and this gene can be influenced strongly by genes that control brown hair. Indeed, two parents with red hair can have kids with brown or blonde hair.
4. Attached earlobes
The myth: Everyone has one of two kinds of earlobes: attached (connecting directly to the side of the head) or unattached (a slight separation causing the lobe to dangle). A single gene decides the fate of your earlobes.
The reality: Our earlobes don’t fall into two categories. Instead, there’s a sliding scale between attached and free. Two of the early studies on attached versus unattached earlobes disagreed on which was the dominant trait, showing that the genetics involved aren't as simple as many have been taught.
5. Hitchhiker’s thumb
The myth: Your thumb is either straight or bent at the knuckle. The latter is called hitchhiker’s thumb, and whether or not you have it comes down to a variation in a single gene. “If the myth were true,” McDonald writes, “two parents with hitchhiker's thumb could not have a child with a straight thumb.”
The reality: There can be no clear-cut definition of a hitchhiker’s thumb because thumb flexibility ranges dramatically from person to person. “It’s completely arbitrary where you draw the line between straight and angled,” McDonald says. Parents with bent thumbs can produce kids with straight thumbs.
The moral of the story? Genetics are complicated. If you really want to see basic genetic traits in action, McDonald suggests looking at cats instead of humans. “Cats do have a number of traits—long versus short hair, orange versus black hair, white boots or not that—that are nice, simple, one gene-traits,” he says. “Everyone either has a cat or knows someone else’s cat.”