Traditionally, genetics lessons have used the shape of the earlobe as an example of a simple trait, a trait that arises from a single gene. Survey a wide variety of ears and decide for yourself if dividing earlobes into two groups—“free” or “attached"—is really so simple.
- Printout (either black-and-white or color) of this sheet of ear cards
- Optional: camera and a way to print out images
- Cut the pictures into individual cards.
- If you want to include your own ear, take a picture of your left ear so the earlobe is clearly visible. Print out the picture, and add it to your deck of cards.
Spread out all the ear cards so you can see them all at once. Each picture shows the ear of a different person. Notice how the shapes of the earlobes differ. What kinds of variation do you see?
Earlobes can be described as “free” or “attached.” Attached earlobes are connected directly to the head, while free earlobes hang down below that point of connection. Organize the pictures into a continuous line, from most free to most attached.
Next, divide the pictures into just two groups: free or attached. Decide which earlobes count as free, and which are attached. Is it easy to decide which image to place into each group?
Which method of organizing the pictures do you think is more accurate for describing the variation you noticed?
Almost all traits have some variation among individuals of a species. To understand how this occurs, scientists first try to describe or measure the extent of the trait’s variation.
Sometimes, it’s most accurate to describe a trait as being discrete, meaning that there are a small number of possible variations, making it easy to assign an individual to a particular group. For example, you might observe that individuals of a particular plant species have either white or red flowers, but never any color in between. Here, when you organize your ear cards into two groups (with either free or attached earlobes), you’re treating that variation as a discrete trait.
Other traits are more accurately described as continuous, meaning that there are many, if not infinite, possible variations. For example, people can be almost any height or weight, and it would not be easy to decide who would count as being tall and who would count as being short in a group of people. Here, when you organize your ear cards into a continuous spectrum, you’re describing earlobe shape as a continuous trait, ranging from free to attached.
Scientists usually consider continuous traits to be complex traits. Complex traits are controlled by multiple genes and alleles, often with additional effects from environmental factors. Because many genes and alleles are involved, it’s difficult to precisely predict the inheritance pattern of a complex trait. The child of two parents of different heights, for example, may be closer in height to one parent than the other, be taller than the tallest parent or shorter than the shortest, or somewhere in between. Height depends on the precise combination of alleles a child inherits, and the environmental conditions under which the child develops.
Discrete traits often turn out to be simple traits, which means that they’re controlled by a single gene, and follow what's known as Mendelian patterns of inheritance. It’s easier to predict the inheritance pattern of a simple trait, as there are only a limited number of outcomes with known probabilities.
While earlobe shape has often been described in biology textbooks as being a discrete, simple trait, you probably had a hard time deciding where to draw the line between free and attached. If you decided that the continuous spectrum of earlobe shape was a more accurate way to describe the variation, you aren’t alone. Family and genetic studies show that earlobe attachment is actually not a simple trait, but rather a complex trait, affected by multiple genes and environmental factors.
Take your own pictures of variation in another human trait, and decide for yourself whether you think it’s simple or complex. You can try looking at the color of people’s eyes, the positions of their thumbs when they clasp their hands together, the relative length of their toes, or whether they have a widow’s peak or cleft chin.
When using this Snack in the classroom, it works well to divide students into pairs or small groups, and give the same set of pictures to each group. You can have the groups do both experiments described here, or you can ask half the groups to organize the pictures into a continuous spectrum, and the other half to organize the pictures into two discrete groups (free and attached earlobe shapes). When they’ve finished organizing the pictures, have students look at the way other groups organized their cards. Did all groups agree on which earlobes counted as being free and which counted as being attached? Did all groups put them in the same order in the spectrum?
Ask students to reflect on what they found difficult about these tasks, and make an argument for which organizational strategy best describes the variation. Can they think of other ways to describe or measure the variation besides putting the cards into a continuous spectrum or two discrete categories?
If you have a way to photograph and print out additional images, you can make a set of pictures for students to examine the variation in the shape of their own earlobes. Other supposedly Mendelian traits (including the ones listed in the Going Further section, above) can be investigated in the same way.
This Snack works well when used near the beginning of a unit on inheritance. It encourages students to view variation in traits with a critical eye, and can be revisited when teaching about Mendelian patterns of inheritance. Many students develop the misconception that a majority of human traits are simple and Mendelian, when the reverse is true. Evaluating the data for themselves can help them understand and notice different types of variation.