Punnett Squares and Pedigrees - Flipped

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SWBAT complete Punnett squares and identify the relationship between genotype and phenotype.

Big Idea

Use this lesson to guide students through the process of completing a Punnett square.


5 minutes

As students enter the room, they take out their journals and begin responding to the prompt: What is a Punnett square?  Why is it used?

While the students work on their journals, I circulate through the room to read their responses.  Some of the students have difficulty recalling the purpose of a Punnett square, so they look for their flipped notes.  I do not have a problem with students using their notes to complete the journal entry.  Ideally, the students will remember the information and not need their notes.  The students' awareness that the answer is in their notes as well as their willingness to seek out information to support their response to the prompt is also valuable.  It demonstrates that they are connecting to the meaning and purpose of note taking. Also, reexamining the notes on their own helps the students become more independent learners.  Since the answer to the prompt cannot be found word-for-word in their notes, the students will still need to synthesize information.  

Once the students have had an opportunity to record their thoughts, I ask them to share their journal with a neighbor.  After they have shared with a neighbor, I ask them to share their responses as a small group.  I then ask a representative from each group to share their answer with the class.  I do not allow the students to say "Our answer is the same as the last group."  I explain my reasoning for this to the students by stating that the more frequently they hear the information, the more likely they are to remember it.


20 minutes

This is the set of flipped notes that students are expected to have viewed and taken notes on prior to this class period.  Based upon the notes, this is a proficient student sample of Cornell notes and a notes review.  This is a blank copy of the Punnett Squares and Pedigrees notes review.  Using Punnett squares as models to describe how sexual reproduction results in offspring with genetic variation addresses NGSS MS-LS3-2.

As the students discussed their journal prompts, a majority of the information from the flipped notes was reviewed.  After examining the students' notes reviews, I was able to identify that the students had difficulty distinguishing between heterozygous and homozygous, so I spent time reviewing the difference between the two.  I have found that reminding students of the meanings of the prefixes of each word helps them to identify their definitions.  

I spend most of this portion of the lesson demonstrating how Punnett squares are completed.  I begin by showing the students a completed Punnett square, as shown on the flipped notes.  I then walk the students step by step through the process of creating a Punnet square.  I draw a square on the board and divide it into sections.  I explain to the students that we are working with genes that have two alleles and that one is dominant and one is recessive, so each of the four sections represents a possible genotype of the offspring.  I then write the parents' genotypes in their respective places outside the square.  From there, I demonstrate various methods for how the students may fill in the squares with the potential genotypes for the offspring and review the process for determining probability.  Using Punnett squares as models addresses NGSS SP2 and having the students analyze the outcomes of the Punnett squares addresses NGSS SP4 (Analyze and interpret data).

I explain to the students that the probability is for each individual offspring, not that if the parents have four offspring each one will exhibit a separate phenotype as listed on the Punnett square, as  I have found this to be a common misconception among students. Reviewing probability addresses NGSS SP5 as students focus on mathematical thinking.

I then hand out the SpongeBob worksheet (created by Tracey Trimpe Tomm and posted on the Science Spot).  I use this worksheet because it starts with basic terminology and builds toward the creation of Punnett squares.  For instance, the page begins with a review of the terms heterozygous and homozygous.  These are terms that my students struggle with initially, so the use of this worksheet helps to address those misconceptions.  I review the instructions with the students and then provide them with a couple of minutes to answer the questions.  While the students are working, I circulate through the room and help students individually, as necessary.  I then review the answers with the students and we proceed to the second section.  It is in this manner that we continue to move through the sections.  

When we get to the fourth section, the section that requires the students to draw a Punnett square, I read the question information with the students.  I then go through the information with the students again, asking them to underline key words and phrases that we will need in order to determine the parents' genotypes.  We then use that information to write the parents' genotypes on the square.  I have the students complete the potential phenotypes for the offspring while I fill in the Punnett square on the board.  Once the students have checked their information, we answer the questions regarding probability.  I lead the students through the next problem in the same manner.


10 minutes

After working through two Punnett square examples together, I have the students complete the rest of the worksheet independently.  While the students work, I circulate through the room, helping students who are struggling with the concept.  I place an answer key on the white board, so students may check their answers when they are finished.  If their answers are correct, they receive an additional practice page.  If their answers are not correct, I ask them to try again and to explain how their original answers differed from the correct answers.

This lesson consists of a review of notes and then worksheet reinforcement as a way to help me prepare my students for a day long DNA/Heredity event in which they will complete various activities with  third grade students.  There are numerous relevant Punnett square activities available to help reinforce the information presented in this lesson.  MT Mariana Garcia Serrato's Furry Family lesson is a great example of a Punnett square reinforcement activity.


Wrap up

5 minutes

Near the end of class, I ask the students if they were able to recognize any patterns in the Punnett squares.  I ask the students to refer to examples from the worksheet to support their ideas.  The students are able to identify that if one of the parents is homozygous dominant, then the offspring will display the dominant trait.  They are also able to identify that heterozygous parents have a 75% chance of having offspring with a dominant trait and a 25% chance of having offspring with the recessive trait.

This discussion of patterns addresses NGSS Cross Cutting Concept of Patterns - Graphs, charts, and images can be used to identify patterns in data.