Phenomenon: Siblings and Sunburns

Consider a pair of siblings raised in the same home. Even though they share the same parents, they look noticeably different from each other. Furthermore, imagine one sibling develops a rare skin condition after a severe sunburn, while the other does not.

Why are no two people (besides identical twins) genetically identical, and how can even identical twins develop different genetic traits over time?

🕒 Estimated Time

• Engage: 10 minutes
• Explore: 25 minutes
• Explain: 15 minutes
• Elaborate / Evaluate: 30 minutes
• Total: ~80 minutes (approx. 2 class periods)

🧰 Materials

• Internet-connected device with access to the Sources of Genetic Variation simulation.
• Student data table (see below).

5E Inquiry Cycle

1. Engage

1. Read the phenomenon above as a class.
2. Brainstorm: List all the possible reasons you can think of that would explain why siblings have different traits.
3. Discuss: If you have identical twins with exactly the same DNA, how could one of them get a genetic mutation in their skin cells that the other doesn’t have?

2. Explore

Access the Sources of Genetic Variation simulation. It contains three modules that model different sources of genetic variation.

Part A: Meiosis (Crossing Over)

1. Locate the first panel: Meiosis (Crossing Over). This simulates chromosomes during the formation of sperm or egg cells.
2. Click on the chromosome segments to swap them between the paternal (blue) and maternal (pink) chromosomes.
3. Observe the “New Combinations” counter.
4. Task: Swap segments to create at least 3 completely new genetic combinations.

Part B: DNA Replication Errors

1. Locate the second panel: DNA Replication Errors. This shows a template DNA strand.
2. Click the “Replicate DNA” button several times.
3. Observe the newly synthesized strand and look for any red “mutated” bases that don’t match the standard A-T, C-G pairing.
4. Task: Click the button until you observe at least 3 point mutations occurring due to copying errors.

Part C: Environmental Mutations

1. Locate the third panel: Environmental Mutations. This shows a DNA sequence inside a cell.
2. Click inside the black environment box to simulate a UV ray striking the DNA.
3. Observe what happens to the bases.
4. Task: Expose the DNA to UV rays until you have caused at least 3 environmental mutations. (You can click “Heal DNA” to reset if needed).

Data Collection Table Copy the following table into your notebook or use the provided handout.

3. Explain

Based on your observations in the simulation:

1. Meiosis: How does crossing over ensure that every sperm or egg cell produced by a parent is genetically unique?
2. Replication: Why might a mistake during DNA copying be considered a “viable error” that creates genetic variation, rather than just breaking the cell?
3. Environment: How do external factors (like the UV rays in the simulation) physically change the genetic code?

4. Elaborate / Evaluate (Deliverable)

Student Deliverable: Argument from Evidence

Make and defend a claim based on evidence regarding the sources of genetic variation.

Your claim should state that inheritable genetic variations may result from:

1. New genetic combinations through meiosis.
2. Viable errors occurring during replication.
3. Mutations caused by environmental factors.

Your argument must include:

• Evidence: Specific observations from the simulation (e.g., how the chromosomes swapped segments, how the UV ray changed a specific base).
• Reasoning: Explain why these three mechanisms result in genetic variation that can be passed on or alter an organism’s traits.
• Connection to Phenomenon: Explain the opening scenario. Which mechanism explains why the siblings look different at birth? Which mechanism explains why one developed a skin condition after a sunburn?

Extension Options

• Research: Investigate a specific genetic disorder and determine whether it is typically caused by a replication error, an inherited trait (meiosis), or an environmental factor.
• Modeling: Draw a physical model of crossing over using colored markers to show how alleles are recombined.

👩‍🏫 Teacher Notes

NGSS Alignment

This task is specifically designed to assess HS-LS3-2: Make and defend a claim based on evidence that inheritable genetic variations may result from: (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.

Science and Engineering Practices (SEP):

• Engaging in Argument from Evidence: Students formulate a claim and support it using evidence derived from the three modules of the interactive simulation.

Disciplinary Core Ideas (DCI):

• LS3.B: Variation of Traits: The simulation explicitly models how crossing over creates new genetic combinations, how DNA replication errors lead to mutations, and how environmental factors (like UV light) can cause viable mutations.

Crosscutting Concepts (CCC):

• Cause and Effect: Students identify the specific causes (meiosis, replication errors, UV rays) that result in the effect of increased genetic variation in a population.

Evidence Statement Mapping

Student work should be evaluated based on the following evidence statements:

• 1.a.i-iii (Developing a claim): Does the student’s deliverable explicitly claim that variation comes from meiosis, replication errors, and environmental mutations?
• 2.a.i-iii (Identifying scientific evidence): Does the student use specific observations from the simulation (e.g., swapping chromosome segments, mismatched base pairs during replication, UV ray zaps altering bases) as evidence?
• 4.a.i-ii & 4.b (Reasoning and synthesis): Does the student logically connect the mechanism to the outcome, explaining how these changes create variations that can be inherited or expressed as new traits (like the skin condition in the phenomenon)?