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Crossing Over Visualization

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About This MicroSim

This MicroSim provides a step-through visualization of crossing over during prophase I of meiosis. Students see exactly which alleles are located on each chromatid and watch as segments are exchanged at the chiasma, producing recombinant chromatids with novel allele combinations.

How to Use

  1. Stage 1: Observe two homologous chromosomes — maternal (pink, A-B-C-D) and paternal (blue, a-b-c-d)
  2. Stage 2: Watch chromosomes replicate into sister chromatids forming a tetrad
  3. Stage 3: See the chiasma (X-shaped crossover point) form between loci B and C
  4. Stage 4: Before seeing the results, you are prompted to predict which alleles each recombinant chromatid will carry. Click "Reveal Answer" to check your prediction.
  5. Stage 5: View the four separated gametes — 2 parental and 2 recombinant — with their allele combinations
  6. Toggle "Double Crossover" to see what happens when a second chiasma forms between loci A and B

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

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<iframe src="https://dmccreary.github.io/biology/sims/crossing-over/main.html"
        height="502px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Grade Level

9-12 (college placement Biology)

Duration

10-15 minutes

Prerequisites

  • Understanding of homologous chromosomes and alleles
  • Basic knowledge of meiosis phases (especially prophase I)
  • Familiarity with dominant and recessive alleles

Activities

  1. Exploration (5 min): Students step through all 5 stages to see how crossing over works with concrete allele data visible at each step.
  2. Guided Practice (5 min): Students use the prediction prompt at Stage 4 to predict recombinant allele combinations before revealing the answer. Then toggle on double crossover and predict those outcomes.
  3. Assessment (5 min): Students explain why crossing over increases genetic diversity and predict the recombination frequency from the four resulting gametes.

Assessment

  • Can students predict which alleles will appear on recombinant chromatids?
  • Can students distinguish parental from recombinant chromatid types?
  • Can students explain why double crossover between adjacent loci reduces the observed recombination frequency?

References

  1. Chromosomal crossover - Wikipedia
  2. Genetic recombination - Wikipedia
  3. Meiosis - Wikipedia