More Punnett Square Practice

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Objective

SWBAT complete Punnett squares and determine probability.

Big Idea

Punnett squares are a topic that some students understand very quickly, while others need extra practice. Use this differentiated approach to challenge all of your students to stretch their Punnett square use.

Engage

2 minutes

As the students enter class, they will take out their journals and respond to the prompt:  On a scale from 1 to 5, (with 1 being no idea), how comfortable are you with Punnett squares?  Why?

While they work on their journals, I circulate through the room to review their responses.  At this point I am looking both at the number they write and the terminology they use to describe their comfort level.  I am looking to see if they are using the vocabulary terms we discussed in the previous lesson.  Using Punnett squares as models to describe how sexual reproduction results in offspring with genetic variation addresses NGSS MS-LS3-2.

Once the students have had an opportunity to record their thoughts, I ask them to look down at their journals and hold up the number of fingers that represents the number they wrote in their journal.  This provides a quick check as to where they are as a class.  This practice is useful because it causes the students to self-assess where they are in their level of understanding.  It also shows me where the students think they are in their level of understanding.  If a student is not feeling comfortable with the material, but is able to complete the problems, I know that I need to provide more support and help them develop a deeper understanding of the material.

Explain

10 minutes

I then ask the students to describe how to complete a Punnett square.  I begin by having one student provide me with information regarding the genotype for the female parent.  This student is responsible for writing the genotype on our class Punnett square.  I then ask another student to provide the genotype information for the male parent and write the information on the class Punnett Square.  I call on a third student to fill in the possible offspring genotypes.  I cold call on the students, so each student should be ready to answer.  Based on their journal prompts, I try to call on students who may struggle a little.  I do not do this as a gotcha, but rather as a way to help walk them through the information.  This must be done very carefully, so the student does not become embarrassed or upset.  I have found though, that helping the student answer the question correctly for the whole class helps to build confidence and understanding.

I then ask the class to write down the probability that the offspring will be homozygous dominant, homozygous recessive, and heterozygous.  We then discuss the possible phenotypes of the offspring.  Using the Punnett squares as models addresses NGSS SP2 and computing probability addresses NGSS SP5 as the students use computational thinking.

Once we have completed the example together as a class, I have the students take out their Chromebooks and open the Punnett Square Online Practice document.  I review each of the websites with the students, demonstrating how each one works. 

This is an overview of the first website the students visit.

I use this website with the students because it provides them with an introductory video that recaps the information we discussed in class and because it provides them with immediate feedback when working through the scenarios.  The students are also able to select from a variety of scenarios, which provides some flexibility. This is a Student work example demonstrating the manner in which I expect the activity to be completed.  Additional interactive websites on this topic can be found on this website.

Explore

25 minutes

I then explain to the students that if they are not comfortable with completing Punnett squares, they should move to one of the front groups in order to receive extra assistance while students who accurately completed the SpongeBob worksheet from the previous lesson could move toward the back groups.  

I sit down with a group of students who are having a difficult time understanding Punnett squares.  I give these students a second SpongeBob worksheet (created by Tracey Tomm) and we work through the problems on the page together, set by set.  When we get to the portion of the page that requires students to draw Punnett squares, I have a large whiteboard that we share and I have individual students complete different portions of the square as we work the problem together.  Some of the students have difficulty understanding the procedure for creating the Punnett square.  After a couple of practice problems where I ask them to complete the squares while explaining their answers, these students are able to move along on their own.  Other students are still struggling with how to represent heterozygous and homozygous alleles.  For these students I use repetition and refer back to the meanings of homo and hetero to help them remember.

I take periodic breaks from the small group to allow them to work independently, while I assist the other students.  I check in to make sure they are not struggling with a concept, and I bring them to the small group if they are.

Wrap up

5 minutes

As we reach the end of class, I focus the students' attention on the last two questions on their worksheet:

Final thoughts - Please explain where you are in understanding Punnett squares.  Are you having difficulty?  If so, with what?  Is this too easy?

How do Punnett squares relate to the other information we have discussed in this unit?  Use the terms DNA, genotypes, phenotypes, and genetics in your answer.

Since many of the students have not had time to answer these questions, we review the answers together as a class.  I focus the discussion toward the second question, unless students have thoughts they want to share about the first.  The students are quick to point out that Punnett squares include genotypes to predict phenotypes.  They have a little more difficulty with DNA and genetics.  Some of them state that a Punnett square is a tool used to study genetics and that DNA is the code for an organism's phenotype.

An alternative option would be to have students complete a CER:  The offspring will.....I know this because.....

Information about how I use the CER process in my classroom can be found in the lesson Drawing Conclusions with CER.