Teaching Heredity

        If you asked biology students about their favorite unit, many would enthusiastically choose heredity. For some, it’s a topic they fondly remember tackling in middle school, and even those new to it often grasp the basics quickly. With a strong foundation, students typically feel confident and eager to take charge of their learning as they dive into practice problems.

           That confidence, however, can sometimes be misplaced. If we focus solely on straightforward question setups and repetitive Punnett square drills, students may miss the deeper, analytical aspects of the unit—or struggle with the complex, wordy scenarios they’re likely to encounter on assessments.
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          In this blog, I’ll share my approach to teaching heredity, blending engagement with inquiry-based strategies to ensure students not only enjoy the unit but also master its challenges.

The Order I Follow

           In high school, students must master both Mendelian and non-Mendelian genetics. To wrap up the unit and bring everything together, they tackle the most mind-bending (but incredibly rewarding!) concept: pedigrees. Here’s the progression I use to structure my heredity unit:

1. Introduction to Heredity (No Punnett squares)
2. Mendelian Genetics - Monohybrid autosomal dominance inheritance
3. Mendelian Genetics - Dihybrid autosomal dominance inheritance
4. Non-Mendelian Genetics - Incomplete dominance, codominance, polygenic traits
5. Non-Mendelian Genetics - Sex-linked inheritance
6. Pedigrees - They serve as the perfect culmination of everything students have learned.

​         In this blog, I’ll break down each lesson, share how I approach it, and explain why I teach it the way I do.

Lesson 1: Start "Slow"

          One key principle I stick to: Day one of the heredity unit is Punnett-square-free. The unit is packed with vocabulary, so I dedicate the first lesson to introducing foundational concepts like chromosomes, genes, alleles, genotypes (homozygous dominant, heterozygous, homozygous recessive), and phenotypes. This lesson, which I call "Introduction to Heredity," is essential for setting students up for success when we tackle Punnett squares later.

          To reinforce these concepts, I assign my Make-a-Monster Activity. Each student receives a unique set of monster chromosomes and uses a digital key to draw their own monster. The result? A classroom full of vibrant, one-of-a-kind creatures that students take pride in. They find this activity "easy"—which is exactly the point. It's a fun, confidence-boosting start to a unit that will soon ramp up in complexity. 😉 

Each student pulls 8 popsicle sticks. The red popsicle sticks are the sex-chromosomes. I keep them in a leftover plastic cat treat bin.

In a class of 34, I didn't have two monsters look the same.

I leave these up for the entire unit.

Lesson 2: Time for Punnett Squares

          Sticking with the philosophy of "slow and steady wins the race," it’s finally time to dive into Punnett squares. Of course, we start with the "standard" dominant/recessive inheritance—no teacher would begin with anything else! Mastering these foundational crosses is crucial, as they form the basis for understanding more complex inheritance patterns. I emphasize to students that while the terms "autosomal" and "monohybrid" might sound intimidating, breaking it down helps them feel more confident. After all, most Punnett square problems are presented in sentences, so students need to become comfortable with the vocabulary to avoid being thrown off.

          To solidify their understanding, I deliver a concise 20-minute lecture followed by multiple-choice practice questions. At my school, our grading platform lets me create automatically graded bubble sheets that upload scores directly into the gradebook. This real-time feedback allows me to quickly identify students who need intervention and those ready to move on to the inquiry-based activity. For differentiation, you may want to give struggling students a non-inquiry (practice-based) worksheet.

          Once students grasp these foundational crosses, they’re ready to tackle something more engaging. Too often, the mathematical nature of Punnett squares causes teachers to shy away from inquiry-based learning in this unit. However, it’s crucial to show students that heredity, like all scientific principles, can be applied to real-world scenarios—even magical ones!

         Why magical themes? They are a great way to navigate the sensitive nature of certain traits and respect the privacy of family structures. So, we journey to a fantasy world for a story-based activity where students take on the role of dragon breeders solving a problem. The issue? Offspring from their pet-line dragons are showing unexpected traits from their military-line dragons—a mix-up that could cost them their business! To unravel the mystery, students must deduce which military dragons secretly mated with the pet dragons, introducing the concept of back crosses in a fun, imaginative way. This activity not only reinforces key concepts but also keeps students engaged by merging science with storytelling.

Lesson 3: Not my favorite either...

       Dihybrid crosses—the part of genetics that makes students' eyes widen with dread. Since these aren't typically covered in middle school, they can initially feel overwhelming. Students need to learn how to determine parental gametes for two traits (using the FOIL method) and interpret a 16-square Punnett square. However, it's important to know they aren’t required to perform these crosses from beginning to end on state tests.

          While it’s beneficial for students to complete a couple of dihybrid crosses from start to finish, the real focus should be on two key skills that are tested: mastering FOILing to determine gametes and identifying phenotypes from genotypes when working with four-letter combinations instead of two. This ensures they grasp the essential concepts without unnecessary overwhelm. 

        I wouldn’t recommend spending too much time here. For example, in a 45-minute class, this lesson would fit perfectly. There’s no need for an inquiry-based assignment—I stick to straightforward practice questions that I’ve created, which are automatically graded and uploaded. This lets me quickly pinpoint students who are struggling and provide targeted intervention before moving on to more advanced material. Any remaining class time can be used for additional autosomal monohybrid practice, reinforcing earlier concepts before we dive into the next, more challenging topics.

Lesson 4 and 5: Sex-Linked Traits & Non-Mendelian Genetics 

       In an ideal world (which I don’t live in), I’d have 45-minute classes for this unit. I’d dedicate one class to non-Mendelian genetics (with practice questions), another to sex-linked traits (also with practice questions), and then two additional 45-minute classes for my aligned activities (details below). This setup works beautifully because both activities incorporate sex-linked and non-Mendelian principles.

       In reality, I teach block classes, which means 90-minute sessions. If you’re in the same situation, I recommend starting with non-Mendelian genetics first. Why? The aligned activity only includes one sex-linked trait at the beginning, so you can guide students through it together before they dive in.

        My non-Mendelian genetics activity is a twist on the Make-a-Monster activity. Instead of monsters, students create troll siblings by performing Punnett squares for troll parents. This activity is fantastic for helping students transition between Mendelian and non-Mendelian inheritance, encouraging them to slow down and carefully consider the differences. It also deepens their understanding of how siblings can share traits due to shared parental chromosomes while remaining unique because of the laws of independent assortment. Bonus: each student works with a unique set of parents, so there’s no opportunity to copy answers!

          For sex-linked traits, I keep it simple and focus only on X-linked inheritance (since I’ve never seen a Y-linked question on state tests). The biggest challenge for students? Remembering to include X and Y chromosomes in their Punnett squares. After finally mastering it, many struggle to switch back to regular annotation for autosomal traits in the next class. That’s why one of the key takeaways from this lesson is learning to carefully read questions and identify phrases like “autosomal,” “sex-linked,” or “carried on the X chromosome.” A well-written question will always provide these clues, signaling whether X and Y annotations are necessary. To reinforce this, I designed my sex-linked practice questions to include both autosomal and sex-linked examples—be sure to explain this distinction to students if you assign my questions.

    To wrap up the sex-linked traits lesson, I use my inquiry-based “Meow-Mendel Genetics Challenge.” In this activity, students pretend they are designer cat breeders. Using real-life cat genetics (albeit, simplified), their goal is to maximize profits by selecting the best two parents (out of six) to produce the most profitable kittens (I made up monetary values for certain traits). With clear right and wrong choices, this activity is not only applicable but also engaging and fun, offering students a memorable way to learn genetics. Plus, who doesn't love cats?

Lesson 6: Finish Strong

          Pedigrees can be particularly challenging for students because they require a deeper level of thinking—backcrossing, test crossing, and analyzing multi-generational data to solve problems. A common misconception is that pedigrees are exclusively for X-linked traits. This likely stems from the frequent use of examples like the Victorian hemophilia family in teaching.

          It’s crucial to emphasize that pedigrees can track any trait—dominant or recessive, autosomal or sex-linked. The shaded symbols simply indicate the phenotype being tracked (so it could be dominant or recessive), and the inheritance pattern depends entirely on the question at hand.
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        Overall, the complexity and integrative nature of pedigrees make them an excellent capstone lesson for the heredity unit, tying together all the key concepts students have learned.

           Try switching things up by saving the multiple-choice questions for last, after the aligned activity. Letting students dive into a fun, creative challenge first can build their confidence with this tricky concept before tackling the more straightforward, disconnected questions.

        For the aligned activity, we’re heading back to the realm of make-believe—or maybe you still believe? 😉 A scout elf accidentally got locked in a toy chest and used the last of his magic to fly back to the North Pole, only to collapse into the snow. As lead doctors in the elf hospital, students must analyze visible traits that gradually reappear as the comatose elf recovers. Using only pedigrees, they’ll track the lost elf’s family and uncover the mystery.

         This activity is one of my favorites! It’s fun, quirky, and brimming with holiday spirit—perfect for keeping students engaged while mastering pedigrees. I teach pedigrees right after Christmas break, but the nature of the story (that he can't get home) makes it perfect for anytime of the year. 

Last Words

            Remember, the key to a successful heredity unit lies in building a strong foundation, scaffolding learning, and encouraging students to explore and apply their knowledge. Teaching heredity is already fun, but let's continue to challenge students to think critically, solve problems, and connect abstract concepts to real-world applications by incorporating a mix of drills and inquiry-based learning.

         If you have any questions about anything, please send me an email! It's [email protected]