Co-dominance and Multiple Alleles

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Students will be able to use Punnett squares to predict the genotypic and phenotypic ratios of offspring when the traits are co-dominant or incomplete dominant.

Big Idea

Dominance is not always complete.


5 minutes

Warm-Up: Which statement describes how two organisms may show the same phenotype yet have different genotypes?

  1. One is homozygous dominant and the other is homozygous recessive.
  2. Both are homozygous for the dominant trait.
  3. Both are heterozygous for the dominant trait.
  4. One is homozygous dominant and the other heterozygous.

This question allows students to activate prior knowledge about genetics and its related vocabulary that they have learned in prior lessons. This question further requires that students connect concepts or ideas to derive the correct answer.

Expect that some students will need assistance recalling the meaning of the terms, phenotype, genotype, heterozygous and homozygous before they are able to respond to the question. If this is the case, first guide students to practice their decoding skills by looking for known word parts to help them determine the meaning of the terms.

Look for students to identify that genotype represents the alleles associated with a trait.  If students say letters, make sure that they know the correct term is alleles.  Look for students to identify that the phenotype represents the physical appearance. 

Allow student to share multiple ideas about which answer is correct and encourage academic discourse around which answer is correct and why.

Introduce New Material

20 minutes

Inform students of the learning target:

  • I can use Punnett squares to predict the genotypic and phenotypic ratios of offspring when the traits are co-dominant or incomplete dominant.

Introduce the vocabulary associated with the lesson: co-dominance, complete dominance, incomplete, dominance, antigen.  Plan to explicitly teach the vocabulary associated with the lesson at the appropriate times within the lesson.

Provide background information based on the previous lessons, Mendel's genetics. Remind students that dominant genes are able to hide or mask other recessive genes if they are both present at the same time.  Emphasize that dominance does not mean “stronger or better” just possesses ability to hide the recessive gene.

Distribute guided notes or use a note-taking format that has been taught.  Provide instruction about complete dominance, co-dominance, and incomplete dominance.

It is likely a student misconception that dominance is always complete so it may help to say “Dominance is not always complete” multiple times and have students repeat it after you throughout the lesson.

Using visual aids show students how to determine the genotypes and phenotypes of parents and offspring for Complete Dominance, Incomplete dominance and Co-dominance.  It is also helpful to provide students real examples and images of Incomplete Dominance and Co-dominance.

Share a video clip on co-dominance and blood types. Note: I like both these videos and differentiate which to use based the needs and abilities of my different class periods:

Multiple Alleles, ABO Blood Types   

 Human Blood Types 

No matter which video chosen, be sure to establish the viewing purpose before viewing so that students will be aware of what they are expected know at the end of the video.  Display the question they should consider while viewing:

  • What do you call the proteins that sit on the surface of blood cells?
  • Why can’t people accept all different blood types?

After the video, give students a chance to discuss the answers to the questions. Instruct them to turn and talk with a seatmate.  Allow 2-3 students to share their responses with the class. Confirm correct responses and correct any errors in thinking.

To reinforce key concepts, display a Rules for Blood Type visual aid that restates information from the video on blood types.  Students benefit greatly from hearing a repeat of information in a different format.  Review the blood type genotypes, as well.

Guided Practice

10 minutes

Display a multiple allele genetic cross Problem 1:

 Show the cross between a mother who has type O blood and a father who has type AB blood.

Perform the calculation while using the think aloud strategy to allow students to hear the thinking used to complete the problem. Note: The visual aid allows me to click the mouse to allow the answers to show on the screen after I think aloud to derive the correct answer.

Display Problem 2 and allow students to guide the calculation:

Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood.

Call on different students in the room to explain what steps to takein order to finish the problem.



Independent Practice

20 minutes

Distribute a set of Co-dominance genetic problems.  Instruct students to use the information provided by blood types to solve the genetic problems. Require students to show their work in order to prove their answers to the problems.  Allow students to work independently or in small groups of two to complete the assignment.   

Walk around the room to monitor students as they work and ensure that all students are actively engaged in the completion of the problems. Listen for misconceptions and guide student thinking for correction, as needed.

After the student work is collected, look not only at the answers but at the Punnett Squares to ensure that students know how to correctly set up the tables to predict the genotypes and phenotypes of the offspring.  The student work sample that is included reflects that the student was able to create a Punnett Square to accurately solve the problems.


5 minutes

Instruct students to visually depict how complete dominance, incomplete dominance and co-dominance are different using colors, symbols or pictures.  Review the completed work to ensure that students are able to demonstrate their understanding of the differences. Student work should reflect that complete dominance results in offpsring that reflect only the dominant trait in the phenotype, co-dominance reflects equal amounts of both traits in the phenotype and incomplete dominance reflects a 3rd new phenotype in the offspring.

Student 1 shows use of colors to convey his understanding of the differences and student 2 uses symbols to convey his understanding of the differences.  While the presentation is different, both students' depictions are correct.