Genetics - Introduction to Punnett Squares

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SWBAT use the basic vocabulary of genetics as it applies to creating Punnett Squares.

Big Idea

How do geneticists determine the probability of inherited traits?

The Need for the Lesson

Students often confuse genetics terms. They need an opportunity to practice the application of terms to build fluency in using the vocabulary of genetics.

Scientists use Punnett squares to determine genetic outcomes for offsprings. Working with Punnett squares give students the opportunity to use the tools of scientists.

Investigation Preparation & Summary

10 minutes

Students will review the language of genetics as it relates to Punnett Squares. A Punnett square show students how genetic variation occurs in sexual reproduction. (MS-LS3-2 Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.)

The Punnett square serves as a model to describe the cross-cutting concept of cause and effect. Students will be developing perseverance as they employ the use of the Punnet square, a tool used by scientists, to predict the resulting genotypes and phenotypes from various combinations of alleles. (SP2 - Developing and using models)

Additionally students will apply mathematics as they use the Punnett square results to determine how likely it is to inherit a trait. (MP4 - Model with Mathematics)  (6.SP.B.5 - Describing the nature of the attribute under investigation)

This lesson was inspired by The original student lesson sheet can be found on the in PDF format.

Trimpe, T. "Bikini Bottom Genetics." Science Spot. C. Trimpe, 2003. Web. Apr. 2015.

Students in Action

45 minutes

Students in Action

We begin the lesson with definitions of genetic terms we will use in building Punnett Squares.


Students have worked with these terms in Genetics - Tour of the Basics so this introduction is a review. The vocabulary is still new to my students the reviews will help move them towards mastery.

We watch How Mendel's pea plants helped us understand genetics, a short TED-ED video, to help students understand how Punnett squares are used to predict outcomes in genetics.

I hand out the student practice sheet for this lesson. I have elected to use a resource prepared by another teacher because it is well done. The practice sheet is broken into sections. We first begin by reviewing vocabulary and then progress to the application of vocabulary as we complete Punnett squares. As a class, we complete the front side of the student practice sheet, then students complete the second page.

As we complete questions 3 and 4, I marvel at the use of the Punnett square as a model that helps us make predictions of genetic outcomes and the probability of occurrence. I share with students that models are an important communication tool in science to help ourselves and others visualize what is otherwise hard to observe.

Students are encouraged to check their work with a neighbor upon completion of the problems. The strategy to check in with a neighbor requires students to have completed the work themselves first. I instruct students that if they find they have different answers, each students should talk through their solution to the question defending their choices. They should begin with reading the question out loud. Sometimes, simply reading the question out loud provides clarity of understanding causing one or both of the students to have an aha moment and rethink their answer. The two students should evaluate the answers and determine how the question should be answered. It is possible that one or neither of the students has the correct answer. Their discussion should lead to a clear understanding of the question and a consensus for the answer.

In this video, I share how I mentor my students as they work on the Bikini Bottom Genetics practice sheet.

Connecting the Learning

5 minutes

I ask my students, How can genetics be so simple?

The answer is that it is not. Mendel's experiments with peas were limited to the combination of recessive and dominate alleles. But there are exceptions to genetics that Mendel's rules do not explain. Alleles are not always dominate or recessive. 

Tomorrow, we will look at incomplete dominance, that is when an allele is neither dominate or recessive but the alleles combine to produce a third phenotype.

Take a look at the flowers. Combining a white flower with a red flower would produce a third phenotype giving us pink flowers.