Incomplete Dominance and Co-dominance

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Students will be able to explore different inheritance patterns beyond Mendelian genetics

Big Idea

Mendel’s laws of inheritance do not account for the expression of all traits.


In this lesson students continue to grow in their 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) collaborating with peers and defending explanations; (SP8) engaging in discussions with scientific peers.

As students work to determine the probability of different crosses, they continue to expand their understanding of the Crosscutting Concept Patterns -"Observed patterns in nature guide organization and classification and prompt questions about relationships and causes underlying them". 

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 close this lesson I ask the students to add the concepts studied today to the genetics concept maps they created during the Tour the Basics-Concept Map lesson. 

Alternatively you could close the lesson by having students write down one or two questions they still have about the concepts studied and review the answers with them at the beginning of the next lesson.


5 minutes

To engage the students in the lesson, I created an Animoto video. 

After I show the video clip once, I move the slider to 0:15 (image of the different dog coats), and ask, "What are you observing? If we follow Mendel's inheritance, should we not have just two colors of dog?". I tell the students to have a brief discussion with an elbow partner (SP7), and to be ready to share out. After about one minute, I use popsicle sticks to choose a couple of students to share their ideas, and write "incomplete dominance" on the board. At this point, ideas range from "there are different alleles for each of the shades" to "it must be because of the environment". A few came up with the idea of selective breeding, but quickly became unconvinced as a peer asked, "How did it come about in the first place?" 

I repeat the procedure stopping at 0:30 (image of a spotted cow), writing "co-dominance" on the board at the end of the discussion. As before, the predominant idea from the students in response to this was "there is a specific allele for spots vs. no spots".


15 minutes

I present this slideshow to support the teaching of the concepts of incomplete dominance and codominance as students take notes individually.

Slide 9 is a check for understanding of incomplete dominance, where the resulting Punnett square should look like:

Slide 15 verifies the students' understanding of co-dominance. A tan cat cannot be heterozygous since the tan color requires two tan alleles. 

 The ratio of phenotypes is two black: two tabby.


20 minutes

I ask students to bring out the monsters they created during the Monster Factory lesson, and distribute the Monster Factory Punnett practice sheet (student work). Throughout the practice, students work with a partner analyzing and interpreting data (SP4), constructing Punnett squares (SP6), formulating evidence based on data, collaborating with peers in searching for the best explanation (SP7), and engaging in discussions with scientific peers (SP8).

As the students are working, I am circulating the room, asking "Why" questions and soliciting explanations for student answers. Watch as students show their work and explain their understanding. Notice the growth in familiarity with the concepts and confidence in their explanations.