Practice with Punnett Squares

This worksheet covers the basics of Mendelian inheritance and Punnett squares. Practice problems, featuring traits from the Mutt Mixer interactive, give students a chance to apply each new idea. By the end, students should be able to fill in a Punnett square for a one-factor cross and calculate the probabilities of offspring having each genotype and phenotype.

You’ll notice that the alleles in this activity don’t follow a standard naming convention that clues you in to their inheritance pattern. That’s because scientists don’t use one. As much as possible, we used the allele abbreviations that are most common in the scientific literature. After all, these are real dog traits.

We recognize that many state tests include questions about Punnett squares and Mendelian traits. And many teachers enjoy teaching with Punnett squares because they can be fun, and it can be satisfying to see students “getting it.” However, we encourage teachers who include this topic to think deeply about their approach. Below the links, we summarize the potential pitfalls of teaching Mendelian traits (based on our own work and that of others), and offer some ideas for an approach that’s more consistent with modern genetics and medicine.

The Punnett Squares Extension introduces three more advanced topics: genes with more than two alleles, independent assortment, and two-factor crosses. If you plan to use Complex Traits: Beyond the Punnett Square, we recommend at least using the independent assortment section.

• Have student work independently or guide them as needed.
• Students will need some information about traits from Mutt Mixer to complete the practice problems. You can have them access the interactive on individual devices or give them the details they need.
• Tip: If you want to add more practice problems, use the Mutt Mixer Quick Reference as a guide to the inheritance patterns for the alleles.

• Punnett squares are models that show the probability of offspring inheriting a particular genotype.
• A genotype is an individual’s allele combination; a phenotype is a visible trait caused by the alleles.

30 minutes

Copies
(optional) Individual student devices for accessing Mutt Mixer

Practice with Punnett Squares (pdf)
Punnett Squares Extension (pdf)
Mutt Mixer (interactive)
Mutt Mixer Quick Reference (pdf)

You may already know a lot about the issues we raise, but maybe it’s been a minute since you’ve had a chance to think about it, and you’d like some fresh and easy-to-implement ideas to address the problems. You’ll find specific suggestions below, under Potential remedies.

Traits that follow Mendelian inheritance pattern are exceptions to the rule. Most traits involve multiple genes plus factors from the environment. Importantly, these include traits that impact our everyday lives, like diabetes, cancer, heart disease, and many others related to health and disease. Contrary to past debates about nature OR nurture, we know today that every trait is a product of nature AND nurture.

Students who learn about inheritance through a Mendelian lens tend to overgeneralize. When students learn about Mendel first, they may think of multi-factorial traits as the exceptions. They are likely to apply what they know about Mendelian traits as they try to make sense of all traits—often leading to false conclusions. Brian Donovan's group has looked specifically at how traditional genetics education can lead to genetic essentialism—the idea that members of a group are essentially the same, or have innate similarities that make them different from other groups. Applied to human groups, these overgeneralizations (many of them unconscious) can lead to racist ideas.

Punnett squares are just one model, and relying on this model alone can introduce very sticky misconceptions. Misconceptions that take root in middle school (and even younger) students persist as they move into college (Jamieson & Radick, 2017) and beyond. Some examples:

• "Your genes are your destiny," or genetic determinism. This idea underplays the role of the environment in shaping an individual’s traits. One danger in this thinking is that students with family members who have certain health conditions may assume they will also develop the condition, and there’s nothing they can do about it.
• "Except for superficial differences, all members in a species are essentially the same." This misconception about genetic essentialism can get in the way of students learning about variation in natural populations and its role in natural selection and evolution.
• "Inheritance patterns for all traits are predictable." A focus on Mendelian inheritance can give students the false impression that all genes come in two versions, one dominant and one recessive. It can also lead to "the gene for X" thinking, which unfortunately is often perpetuated in media coverage.

The terminology and mechanisms around dominant and recessive get muddy and confusing. The language of Mendelian inheritance contributes to many misconceptions, such as the false ideas that dominant versions of a trait are better, stronger, or more common, or that there’s a specific mechanism by which one allele “dominates” another.

In reality, whether an allele is dominant and recessive is a matter of perspective. The terms not only tend to break down, they also apply to very few traits. To learn more, watch What Really Makes a Trait Dominant or Recessive?

Mendelian-focused teaching can also instill the false idea that single genes are important for building certain traits—and these traits are affected by no other genes or environmental factors. Another way to look at Mendelian alleles is as powerful "difference makers" that, in a very particular context, cause pronounced and specific trait differences.

If you teach Mendelian inheritance, spend at least as much quality classroom time on complex traits. After your students work with Punnett squares, it’s important to then extend those concepts to help students understand how multiple genes and factors from the environment can contribute to complex traits. Better yet, you can start with complex traits (see Jamieson & Radick, 2017) that are familiar to your students, like height, personality, or diabetes—then move on to how individual genes are inherited.

Think of other ways to model the basic laws of inheritance. Punnett squares can be useful for teaching the basic laws of inheritance—but they’re just one model. There are other ways to show that genes come in pairs and one gene in each pair comes from each parent. And other models can better show the idea that both genes in a pair contribute to an individual’s traits.

Consider how you can vary environmental factors to produce a continuum of trait variation. For example, a gene that affects height in pea plants follows a Mendelian inheritance pattern under certain conditions. But if you grow plants with the same genotypes in different environmental conditions, or look at plants of different ages, they will have different heights. Similarly, by varying the growing conditions of plants with different genotypes, you can cause them to have the same phenotype.

Talk about how even supposed single-gene traits are influenced by other genes. For example, human eye color is often taught as a simple Mendelian trait where brown is dominant and blue is recessive. But when you look at actual eyes, there’s a lot of variation in what ‘brown’ looks like. Plus, eye colors don’t always fall into neat bins. And what about green eyes? Again, it’s better to think of this one gene as being an important difference maker among many that help build an eye.

Teach about risk, especially as it relates to complex diseases. When students understand that inheritance patterns for most traits can be unpredictable, and the environment’s role in shaping traits, they are empowered to direct their own destiny.

Every teacher wants to arm their students with the skills and information they need to make important life decisions. Toward this goal, it’s important for teaching to reflect recent shifts in medicine and genetics research. It’s time to move past Punnett squares and “gene for” thinking, preparing students instead to understand modern genomics and the underpinnings of complex traits. While it’s important to understand our history, it’s also vital to move on from past reductionist ideas and toward the exciting research and discoveries of today.

Jamieson, A., & Radick, G. (2017). Genetic determinism in the genetics curriculum. Science & Education, 26(10), 1261-1290. [link]

Boerwinkel, D. J., Yarden, A., & Waarlo, A. J. (2017). Reaching a consensus on the definition of genetic literacy that is required from a twenty-first-century citizen. Science & Education, 26(10), 1087-1114. [link]

• Table 7 includes a helpful list of misleading gene-trait ideas and more-complete ideas to replace them.