science lesson

As soon as I start my heredity unit with my middle school students, someone asks the question, “Why do I have blue eyes and both of my parents have brown eyes?” Without fail. It’s not a super simple answer when they’re just learning about heredity. Students have to learn how to complete a Punnett square. I am always so excited to show them Punnett square examples of how this eye color phenomena actually happens!

Some kids really get it and others looked at me like I am an alien. It took a couple years for me to realize teaching Punnett Squares can be tricky because they seem so simple. When I really took a look at why some kids were struggling, I realized Punnett squares are a little more complex that I originally thought.

So many things that can go right and wrong. Even kids who complete the Punnett square correctly don’t always know what the information is trying to tell them. So let me help walk through a few tips and tricks that have helped me! 

What Are Punnett Squares?

Punnett squares are a diagram showing us all the possible genetic combinations of offspring created by two parents for a specific trait. In other words, it helps us predict the likelihood offspring will have specific traits. Can two parents with brown eyes really have offspring with blue eyes? Simply, yes. Getting kids to the place where they can answer this on their own takes a bit of work.

The Secret Sauce to Students Getting It

There’s a strategy to teaching Punnett squares in a way that makes sense to your students. The last thing you want to do is start by working through several Punnett square examples. Although this works in theory, there’s a lot of background students need to complete and collect information from Punnett squares.

Focus on Vocabulary!

Do not skip this! Kids need to know a lot of vocabulary to learn Punnett squares well. Strong vocabulary knowledge gives students the ability to set up simple and complex Punnett squares and the understanding they need to analyze the information they find. 

I’d suggest teaching the words trait, offspring, allele, dominant, recessive, homozygous, heterozygous, dominant, recessive, genotype, and phenotype. 

Spending time on this vocab will result in stronger science students who are ready for high school! 

Don’t be the teacher who moves right from vocabulary to Punnett square examples. There are so many small mistakes students can make that will really throw them off later, but being able to make sense of Punnett squares in their head before you actually teach it helps so much! Click here to find this investigation and more resources to teach Punnett squares!

Be intentional about Punnett Square Practice Problems

From setting up Punnett squares to analyzing what kind of information they’re telling us, be intentional about how you work through Punnett square examples. Scaffold each step and be sure your students are ready to move on before introducing the next skill. Don’t just give them Punnett Square practice worksheets. Guide them through every step.

Punnett Square Setup

I’m going really elementary here. Ready? Draw a square and then, starting in the middle and a little bit above the top, draw one line vertically separating the square into two equal rectangles. Next, starting a little to the left and in the middle, draw a line horizontally. This creates four smaller boxes and splits both the top and the left side into two sections as well. 

One set parent allele goes on the top; one letter above each box in the outside spaces. The other set of parent alleles goes on the left side; one to the left of each box in the outside spaces.

How Do You Do Punnett Squares?

Each one of the parent alleles on the top of the Punnett square falls and is written into the two boxes below it. Then, each letter from the side scoots over and is written in the two boxes to the right. It’s kind of a drag and drop. 

Your capital letter is the dominant allele and the lowercase letter is the recessive allele.

Another great way to explain this is to tell students to think of it as the distributive property in math. When you have 2(4x+y), you distribute the 2 outside of the parentheses into the 4x and the y. This turns into 8x+4y. Students distribute the letter outside of the Punnett square, the parent allele, into each box. You can see this in the color coded GIF!

Be sure to tell students the capital letter always is first in an allele pair. No matter what. 

So, What Do Punnett Squares Actually Say?

Once we’ve ended up with two alleles, one from each parent, in each box and completed the Punnett square, we can answer the question: what are Punnett squares? These alleles are the possible allele combinations, or genotypes, the offspring can inherit from the parents. 

Genotypes are different from phenotypes. Genotypes are the allele combinations, while phenotypes are the physical expression of those alleles. While a Punnett square can have BB, Bb, and bb genotypes, it only shows physical expression of brown and blue eyes. 

The example we walked through shows you why vocab is so important! Students really need to know how all these words work together in order to complete and draw information from Punnett squares.

Kids Still Struggling? Here’s some troubleshooting help!

While students are working, pay attention to what they’re doing. It’s easy to sit and grade or answer emails. Don’t do it! Until your kids have a good understanding, walk around, check work, and correct as needed. I explain how why scaffolding is so important in my blog post Five Easy Tips To Teaching Heredity In Middle School.

These are some issues I’ve run into while teaching Punnett square examples and how to remedy them.

Students only put one allele inside of each box.

It’s possible that there was some miscommunication (or not listening) when students were learning where to place alleles on the Punnett square. If you see this with a few kids, pull them over and reteach or do a few more examples. If you see this a lot, reteach everyone. Do more examples together. Employ the I do, we do, you do strategy. Use whiteboards (maybe even in pairs) for quick checks. Make practice stations where struggling students can work with and listen to conversations their peers are having.

Students don’t know how to analyze a Punnett square.

For whatever reason, teaching how to read and analyze a Punnett square is overlooked. Be sure to scaffold your Punnett square instruction. Complete a few simple Punnett squares, then ask what genotypes and phenotypes a few Punnett square examples show. Next, ask something like, “what percentage are heterozygous?” Or, “What is the possibility that the offspring will have freckles?” and specifically teach students how to find answers to those questions. We often forget that we’ve been doing this so long it’s easy! Students are brand new to this. Give them the proper support to be successful.

Students are just lost.

Remediate vocabulary! Punnett squares are so vocabulary heavy and kids really, really need to have a good grip on it. If you find most of your students are struggling overall, reteach vocabulary. Give each word hand gestures or have them create a game. If most of your students are lost, don’t push it. Go back and reteach, starting with vocab.

I’m a huge proponent of student investigation in science. I think it’s so valuable. But there’s also value in direct instruction. If your students are struggling, sometimes the best thing to do is just tell them what to do. I can set up the coolest investigations for seasons, moon phases, and tides, but at the end of the day, there will always be a kid who tells me the earth is flat. And you can let kids investigate Punnett squares, but you’ll still have to follow up by teaching some of the information directly. It’s okay to tell kids the right answer. 

Back to our question, can two parents with brown eyes really have offspring with blue eyes? Absolutely, they can! Don’t tell students though. Be intentional as you work through Punnett square examples with them. Then they’ll be able to find the answer out for themselves!