Mendelian Genetics - Introduction to Punnett Squares

54 teachers like this lesson
Print Lesson


SWBAT use Punnett squares to predict genotype and phenotype of offspring in genetic crosses.

Big Idea

Introduction to use of Punnett squares.


1 minutes

This lesson address the following standards:

NGSS Standard:

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. 

This lesson introduces students to Punnet Squares which visually represent how sexual reproduction can results in offspring with genetic variation. 

Science and Engineering Practices:

SP2:  Developing and Using Models

SP3:  Planning and Carrying outI Investigations

Crosscutting Concepts:

Patterns (genetic ratios patterns can be used to predict genetic cross results) 


5 minutes

To engage students in this lesson they complete a quick read of an article dealing with selective breeding. Selective breeding is a real life application of Mendelian Genetics and in this case breeders intentionally cross plants to create offspring with desired traits. Breeders use genetic ratios along with probability in their crosses. Breeders need to understand how to use Punnett squares to determine these ratios.

I used two articles from NewsELA* to engage students.

  1. Pumpkins from another planet? No, Wisconsin
  2. The search for the next "rock star" apple

Students complete Marking the Text as they interact with text.

Annotating a text, or marking the pages with notes, is an excellent, if not essential, way to make the most out of the reading you do for rigorous courses. Annotations make it easy to find important information quickly when you look back and review a text. They help you familiarize yourself with both the content and organization of what you read. They provide a way to begin engaging ideas and issues directly through comments, questions, associations, or other reactions that occur to you as you read. In all these ways, annotating a text makes the reading process an active one, not just background for writing assignments, but an integral first step in the writing process.

1.  Number paragraphs

2.  Circle Key Words

3.  Underline author's claims and other relevant information.

After reading I explain to my students that they, just like the plant breeders in these articles, will be learning how to use Punnett squares to predict genetic make up genetic crosses.

*NewsELA is a free site, some access is available without setting up an account. 

Guided Practice

15 minutes

In this section of lesson I start by going over a Genetics powerpoint presentation. (Included in the resources as a Genetics Handout as well, in case you want to make student handouts).

This presentation reviews and introduces new concepts to our genetics unit. These include:

1.  Heredity

2.  Gregor Mendel's Work

3.  Genes and Alleles

4.  Dominant vs Recessive

5.  Punnett Squares

6.  Phenotype vs Genotype

7.  Homozygous vs Heterozygous

I explain in detail how to use this powerpoint in my Visual Learning Reflection.

Students take Cornell Notes during the presentation.  

The Cornell note-taking system originated at Cornell University based on research done in the area of memory and learning theory. It is a very valuable system because it takes students through the cycle of learning. It is much more than just a way to record information. Teaching Cornell note-taking will take time, however it will be one of the most valued skills that students will take with them when they enter college  

The Cornell note-taking system incorporates what students do with their notes once they have taken them. By using Cornell Notes consistently, students learn to see writing as a tool for learning in mathematics as well as other subject areas. The Cornell note-taking system is a system that students need to be taught and reviewed regularly. It is not intended to change how teachers deliver information, but rather how students record and interact with that information. 

This video introduces and explains how to take Cornell Notes.


Teacher Notes:  Prior to this lesson I have already covered Concepts 1 - 4.  In this lesson I focus on the remaining concepts (Punnett squares, phenotype vs genotype, homozygous vs heterozygous).  

The Genetics powerpoint contains formative assessments on all concepts.  It is imperative that students have a good grasp of the new vocabulary (concepts 6 and 7) to be successful in completing Punnett square problems.

I have included supplemental resources that I use depending on assessment results:

1.  Genotype/Phenotype (handout assesses student understanding of these concepts).

2.  Punnett 1Punnett 2Punnett 3Punnett 4Punnett 5 (Interactive Punnett Square Practice from The ScienceSpot.)

Independent Practice

25 minutes

Students independently practice using Punnett squares, by completing the following handouts from The ScienceSpot.

Teacher Note:  I model one problem from each major section of handout to get students started on the right track.  In terms of the Punnett square problems, I teach students to circle/highlight all vocabulary or clues.  I explain to students that they are detectives and before they can solve the problem they must gather as much evidence from the scene as possible.

As students start working on their problems I circulate around the classroom and provide students with one on one feedback.  In instances that I feel the majority of students are stuck on a problem I pause class and I clarify it on the board.  The key to these problems is not so much the actual process of completing the Punnett Square but rather having mastery of the vocabulary (i.e. dominant/recessive, hybrid/purebred, homozygous/heterozygous).  Students are instructed to have their Cornell Notes out in class.  In addition students use my Genetics Word Wall to assist them with vocabulary

Independent Practice Worksheets: (Depending on level of class you may want to complete one practice worksheet daily rather than a complete set all at once).

1.  SpongeBob Genetics 1 (homozygous/heterozygous,phenotype/genotype, Punnett squares)

2.  SpongeBob Genetics 2 (homozygous/heterozygous,phenotype/genotype, Punnett squares)

3.  SpongeBob Genetics Quiz (homozygous/heterozygous,phenotype/genotype, Punnett squares)

Extension/Enrichment Worksheets:

1.  SpongeBob Dihybrid Cross

2.  SpongeBob Incomplete Dominance


15 minutes

I assess students by having them complete a Monster Genetics Lab. This lab assesses student proficiency of all concepts covered in lesson. (SP2 - Developing and Using Models/Planning and Carrying Out Investigations

The activity allows students to apply their knowledge of the simple and complex genetic traits. Students demonstrate how they are able to apply and synthesize what they have learned in a fun activity. 


1.  I modified three traits due to them requires students to have knowledge of three patterns of inheritance that students aren't responsible for in the 7th grade:  incomplete, codominant, regulatory.  These traits are Eye Color, Skin Color, and Tail.  I modified by changing them to a simple dominant/recessive pattern of inheritance.

2.  Since the purpose of this activity was to assess students mastery of Punnet Squares I didn't have students create a drawing of their monster.  Students completed a similar activity in my Create a Genetic Composite Lesson.