Genetics - Incomplete Dominance

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SWBAT explain using the Punnett square model how some alleles can be neither dominate or recessive.

Big Idea

Incomplete dominance is the blending of dominate & recessive traits.

The Need for the Lesson

Students learn about Mendel's genetics where one allele is dominate over another. But there are exceptions to Mendel's rule for heredity. Not all traits are determined by simple combinations of dominate and recessive genes. Incomplete dominance is a simple example of more complex heredity to share with middle school students.

Investigation Preparation & Summary

5 minutes

Students will review the language of genetics as it relates to Punnett Squares. In this lesson Punnett square show students how genetic variation occurs in sexual reproduction when dominate and recessive traits are blended to produce a new phenotype. (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 to predict the resulting new phenotype 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

I begin this lesson with this display of flowers. 

In the last lesson, Genetics, Introduction to Punnett Squares, we concluded with this image. Some traits are blended when combined. Incomplete dominance is one of the ways a variety of flowers are created. Mendel's laws helped us create Punnett squares where alleles were either dominate or recessive. 

Today we will revisit the residents of Bikini Bottom as we explore how to use Punnett squares to predict the outcomes when two traits are blended that is there is incomplete dominance and a combination of alleles creates an intermediate color. In this example, white flowers and red flowers yield genetic offsprings that are pink.

I continue this lesson by working through the first three problems with students. The problem set is introduced with a story about flowers. In this case red and blue flowers, which when crossed produce purple flowers. This is not unlike the example I have already shared with students. We determine the genotypes for each of the colors or flowers. Questions 2 and 3 asks us to complete the Punnett squares for different cross of Poofkin flowers. We review the definitions of genotype and phenotype for each problem. These are vocabulary words that I want my students to master so I really appreciate that each question asks the students to identify the genotype and phenotype for the offspring.

I ask the following questions to clarify student understanding. 

What does the Punnett square tell us? The genotypes of the offspring.

How do you know the phenotype? We determined which letters to use in question 1.

The next questions probe student understanding of probability. I want to be certain that they understand that the Punnett square is a prediction over time.

What other information does the Punnett square tells us? The probability of each flower color.

Is it possible in question 3 that we could cross the purple flowers and end up with only purple offspring? Yes, the Punnett square only predicts the probability for offspring, not what actually happens.

How is predicting probability using Punnett squares like flipping a coin? When you flip a coin, say 10 time, you might have 7 heads or tails in a row instead of having an equal number of heads or tails. You have to flip the coin a large number of times to see exactly 50% heads and 50% tails. The Punnett square predicts the results over time.

I challenge students to complete the rest of the problem set themselves, then check their work with a neighbor. I walk around the room as they are working and answer clarifying questions. When I speak of clarifying questions, I am expecting that students will ask specific questions and not give a blanket statement such as "I do not understand any of this!" If students do make such a statement, I redirect them to show me what specifically they do not understand. Often students will start explaining what they think they do not know and before they finish proclaim that they get it now. This problem set does require careful reading.

In this video I share some of my favorite things about the organization of the student practice sheet.

Connecting the Learning

5 minutes

I tell students that there are other exceptions in genetics. As they pursue their education in biology they will learn about others.

I often provide links to additional resources for my students that might be interested in exploring more about genetics. Here is an example from the Concord Consortium, where students can explore heredity and genetics by breeding and studying virtual dragons. Students can play this game as a guest. Although this simulation is targeted to high school students, the lure of dragons was pretty powerful with several of my students. They all suggested that next year I include this simulation as one of the lessons for genetics!


Link to Geniverse