Misconception Monday: Gimme a B! Now Gimme a b!

It’s getting harder and harder to come up with new misconceptions to cover here. Not because there aren’t more out there, but because misconceptions about evolution overlap significantly and we’ve covered enough of them now that finding one in virgin territory is getting more and more difficult. As a result, I’m looking everywhere for inspiration. At lunch last week, I found some—two young mothers in an adjacent table were discussing their children’s eye color. Where did the baby get her blue eyes? one wondered. The other said that she thought she remembered from school that if one parent has brown eyes and one has blue eyes, the children should all have green eyes, not blue, so they declared it a mystery. I looked despairingly at my husband, but he whispered to me, “just eat your food.”


Misconception: Traits are determined by single genes with two variants.

Correction: Traits are often determined by a complicated mess of multiple genes interacting with each other and the environment.

If you’re over the age of about 30, you likely did the Punnett square for eye color in a biology class somewhere along the line. You were taught that the eye color gene came in two varieties: B and b. “Big B” was dominant over “little b” so that the genotypes BB and Bb resulted in “brown” eyes (“brown” here meaning anything but blue) while blue eyes resulted only from the bb genotype. I’m sure this caused most of you to do your own little squares to figure out the possible genotype combinations of you and your parents and at least some of you to wonder where on Earth your brown eyes came from since both of your parents had blue eyes! Well, no need to cross-examine the mail carrier—it turns out that eye color, like so many other things, is not that simple: There are at least three genes involved and at least nine categorical eye colors that can result from how the variants of each of these genes interact. In short: BB x bb = Bb is a vast oversimplification of how eye color is inherited.

But so what? Why does it matter if students think that things are simpler than they really are? Don’t they have to start somewhere? Yes! Absolutely. I wouldn’t advocate for the discontinuation of classroom Punnett square activities—but I would (and do often) suggest using hypothetical examples, not pseudo-real ones. Your students can practice Mendelian principles of genetics using robots with two types of antennae, or giants with one eye or two, or dragons that breathe fire or don’t just as easily as they can with any oversimplified “real world” example.

Now wait a second, you may be thinking, is Stephanie really advocating for teaching the genetics of dragons and giants in biology class? Well, I guess so. But I really don’t see the problem as long as you’re clear that it’s a model to help understand how the real world works. If you’re uncomfortable with the fantastical, though, there are plenty of “real world” examples that do work, too, if you’re willing to talk about peas and diseases…a lot.

Once you get the basic Mendelian rules down, you can return to real, observable examples. Have your students map out a generation or two of eye color, and turn it into an opportunity to explain that most traits, like eye color, do not map 1:1 with genes. From there, you can bring the point home by conducting a class survey of observable traits and mapping out their distribution. It should become clear that there is not a single “height gene” or “hair-color gene”—but, importantly, that doesn’t mean that the genes that are contributing to these phenotypes aren’t following basic inheritance patterns! They are; it’s just that each gene has a pattern, and the effects of all the genes contribute to one observable phenotype. Sure, BB x bb = Bb is easy to understand and works for a certain limited subset of traits—but I contend that we’re not doing anyone any favors by suggesting that things are simpler than they are. The truth isn’t that much more to take on, and it’s backed up by evidence, which will make it easier to understand and appreciate in the long run.

Another important consideration is that Mendelian genetics lays the foundation for students to understand population genetics. Understanding that continuously varying traits—from finch beak size to susceptibility for certain diseases—have a genetic basis is critical for understanding how natural selection changes these traits in populations.

Plus, the truth involves a little bit of mystery—sometimes, even when the genetics of a trait are understood, things don’t always show up as you’d predict, thanks to complicated interactions with the environment (both the internal environment and external environment). Who doesn’t love a good mystery? You can charge your kids with one day unraveling these interactions so that we can understand once and for all where our eye color comes from.

Are you a teacher and want to tell us about an amazing free resource? Do you have an idea for a future Misconception Monday or other post? See some good or bad examples of science communication lately? Drop me an email or shoot me a tweet <at>keeps3.

Stephanie Keep
Short Bio

Stephanie Keep is the former Editor of Reports of the National Center for Science Education

We can't afford to lose any time when it comes to the future of science education.

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