Meiosis, part 2 - Modeling Crossing Over

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Students will model the process of crossing over in order to better understand how genetic variation results during meiosis.

Big Idea

Hands-on activities help students understand how variation occurs as a result of crossing over.


5 minutes

Warm-Up: “What is the significance of having sex cells with a haploid chromosome number?

Activate student’ prior knowledge from the previous lesson, Meiosis, part 1. Be prepared to prompt students with questions like, “What does haploid mean or what does diploid”, as a means to get them thinking before they attempt to respond to the question. This question serves as a great way to lead students into a quick review. Ultimately, look for students to be able to communicate that haploid cells are significant because organisms require chromosomes from both parents and the haploid number in the sex cells allows the chromosome number to remain the same. Look for students’ ability to use the content vocabulary in their responses. 

Content Review

5 minutes

Don’t spend a lot of time reviewing concepts at this point.  The Guided Practice activity will serve as a more complete review later in the lesson.  The animation serves primarily as an aid to help students visualize the crossing over process so that they will be better equipped to complete the hands-on modeling activity that will follow.

Share a brief animation, Meiosis: Crossing-Over.

  Be prepared to stop the video clip at different points to ask questions:

  • What does sister mean in this context?
  • What is a dyad?
  • What is a tetrad?

Look for students to identify that sister means that the chromatids are a part of the same chromosome, with the same traits coded on each. Having seen the images of a dyad and tetrad in the video clip, students should also be able to explain that dyad means 1 pair of homologous chromosomes that result when a tetrad divides and tetrad means 2 homologous chromosomes.

Guided Practice

10 minutes

Display the crossing over diagram. Distribute Crossing over diagrams and markers to students. Instruct students to select two color markers for the assignment and color each of the 2 chromosomes a different color. Allow them to work independently before sharing an example of the color pattern for the 2 chromosomes.

Using an LCD projector and interactive pad, guide students in the completion of the diagram. 

Lead a discussion of each of the 9 numbered places where responses are needed on the diagram. Ask questions that will lead students to identify the correct responses to each:

  • Item 1- These two chromosomes code for the same trait at the same place but are from two different parent organisms.  What can we label item 1?
  • Item 2- What do we call the point of attachment between two chromatids?
  • Items 3 and 4- One chromosome is from a male and the other from a female.  What can we label items 3 and 4?
  • Item 5- How can we describe homologous chromosomes/
  • Item 6- What part of this image is the chromatid? What makes these chromatids sister?
  • Item 7- What do we know about crossing over from the video clip and yesterday’s lesson that will help us create a short description of crossing over?
  • Item 8-Notice the arrows.  This entire diagram represents what?
  • Item 9- Variation means what?  Let’s think about everything we know about crossing over to decide how it relates to genetic variation or differences.

Walk around to ensure that students are adding the correct information to their diagrams. Explain that students will need to refer to this diagram during the modeling of crossing over during the Independent Practice that will follow.

Independent Practice

20 minutes

Prepare bags of pipe cleaners, sections of straw and beads in advance. Distribute the bags of lab materials. Display and distribute copies of the Crossing over Lab.

Summarize the lab procedure and explain that students will work in groups of two to complete the modeling activity. Read the questions aloud before releasing students to work on the activity. This will serve as a preview of what they are expected to observe and know as they work.

Release students to work together to create the chromosomes. Walk around and note if students are able to correctly make a chromosome, using the pipe cleaners and beads. The student work sample indicates that students know the content.  

As students add traits to the pipe cleaners, ask questions to make sure that students know what the beads represent and why they are the same colors on the same pipe cleaners. Look for students to note that the beads represent the traits and that they are the same color because they exist as a part of a homologous chromosome.

The student work that is attached shows that the students is able to label all parts of the diagram, as well as respond correctly to the lab questions.


5 minutes

Engage students in a discussion of question 10 from the lab, “What significance is crossing over for evolution of a species?” Listen for evidence of understanding that genetic variation is what creates adaptations within a species.

This question should elicit thoughts and comments that demonstrate that students understand how adaptations within species are the result of the genetic variation that arises from crossing over.