Punnett and The Rules

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Students will be able to set-up and complete a Punnett Square.

Big Idea

Traits are defined by directions encoded in genes.


In this lesson students gain an understanding of several Disciplinary Core Ideas:

  • LS1.A: Structure and Function – All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins.

  • LS3.A: Inheritance of Traits – Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species' characteristics are carried in DNA.

  • LS3.B: Variation of Traits – In sexual reproduction, meiosis can create new genetic combinations and thus more genetic variation.

Students will engage in several Scientific Practices – (SP2) developing and using models; (SP4) analyzing and interpreting data; (SP6) constructing explanations; (SP7) engaging in argument from evidence.

As students work independently to determine the probability of several crosses, they will have an opportunity to explore the Crosscutting Concept of Patterns - "Patterns in rates of change can provide information about natural systems." Students will be using patterns to identify cause and effect relationships and use Punnett squares to identify patterns in data.

This lesson only touches briefly on probability (7.SP.C.5) as the students determine the likelihood of the appearance of a specific trait. 

Note to Teachers

This lesson assumes that your students participated in the Monster Factory lesson. They will use the monster they created during that lesson to complete this lesson's work. 


5 minutes

To start the lesson I ask the students to bring out the monsters they created the day before. I display my monster and ask, "Which of the monsters you created yesterday would you cross to try to get my monster? Why? Talk it over at your tables, come to a consensus and be ready to explain your answer."


After the discussion, I have a "reporter" from each table (selected at random), to share out.

I then state that the purpose of today's lesson is to understand how to use a Punnett square to predict the probability of obtaining specific traits in a cross between two organisms. 

This simple use of the previous day's material helps to set the stage and usefulness of the content that will be discussed today. It allows students to organize their thoughts and construct explanations based on evidence (SP6).

I Do

15 minutes

I begin the slideshow, explaining Mendel's rules of genetic inheritance.

Slide 6 includes a mini-movie that I created, walking the students through the DNA from the Beginning website (Topic 5). I chose to walk the students through the animation instead of having them do it on their own so that I can add more detail to the explanations.

As I continue on to slide 7, I review the term independent probability. It is imperative that the students know that each each offspring is the result of an independent event, lest they leave the classroom thinking that the pairing of gametes for a specific trait depends on the number of crossings.

As we move on to Slides 9-11, I am explaining how to set up and complete a Punnett square, and in slide 13, I explain how the tool helps determine probability of a particular trait.

We Do

10 minutes

As we continue to go through slides 14 through 16, I slowly release the students. This means that I will start asking students for help in filling in the sample Punnett squares, and figuring out the probability of a trait in a crossing. It is key to give time to students to think and have them explain their answers as we are working through the examples.

Some sample questions I use:

- What is the probability of ___? 

- How can you tell that in this crossing there is a zero probability of ____?

- (In slide 15), why are there no "medium stem" plants? What would you do to try to get a medium sized stem? (Make sure that you clarify that crossing a tall and short plant will not get a medium plant)

- How many crossings would I need to get a ____? (Each event is independent)

You Do Together

20 minutes

I distribute the Punnett practice sheet, and tell students that they will be working on this with an elbow partner, and will only be "grading" one sheet from each table, so their job is to talk to each other as they are completing the sheet, ensure that everyone has an opportunity to share their ideas, and that all answers make it to every sheet. Their answers to the practice (SW), and to the challenge based on the monsters they created (SW1SW2) reveal that they understand the concepts and are able to determine probability of traits, although they need practice in explaining their answers. Watch as some students work through the practice sheet and reveal their understanding.

The sheet is designed to have students practice analyzing and interpreting data (SP4) and constructing explanations (SP6) as they predict the probability of the crosses. The "one grade strategy" reinforces the need to engage in discussions with scientific peers and evaluate the validity of the findings of others (SP8).


5 minutes

The exit ticket or deliverable for the day is a post it note on my reflective chart. This chart is posted at the front of the room, and gives me a quick reference into student thinking and attitudes in the classroom.