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STR Electropherogram Comparison

Run the STR Electropherogram Comparison MicroSim Fullscreen

About This MicroSim

When a lab profiles DNA, it doesn't read the whole genome — it reads a handful of short tandem repeat (STR) sites called loci. At each locus, everyone carries one or two alleles, and an allele is just a count of how many times a short sequence repeats. Capillary electrophoresis turns those counts into an electropherogram: a row of peaks whose left-to-right positions equal the repeat numbers.

This MicroSim, investigators, puts the crime-scene profile (blue) next to a suspect's profile (orange) at five CODIS loci — D3S1358, vWA, FGA, TH01, and D8S1179. You choose which of three suspects to test. Exactly one suspect matches the scene at every locus; the other two differ somewhere. Press Compare Loci and the tool tallies the matching loci, flags the first locus where a suspect differs, and returns an inclusion or exclusion — the same locus-by-locus logic a real analyst uses, where a single mismatched locus rules a person out.

How to Use It

  1. Pick a Suspect (A, B, or C) from the first dropdown.
  2. Use the Locus dropdown to focus one locus at full height, or leave it on All loci to see all five stacked traces at once.
  3. Press View to switch between Overlay (scene and suspect on the same axis) and Side-by-side (scene on top, suspect below).
  4. Read each locus: do the suspect's orange peaks sit at the same repeat numbers as the scene's blue peaks?
  5. Press Compare Loci to tally matches. A green ✓ or red ✗ appears on each locus, and the banner reports 5/5 loci match — inclusion or differs at locus X — exclusion.
  6. Test all three suspects to find the one that cannot be excluded.

Learning Objective

Analyze crime-scene and suspect STR electropherograms locus by locus to determine how many CODIS loci match and decide whether to include or exclude each suspect — recognizing that one mismatched locus is enough to exclude (Bloom Level 4 — Analyze).

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/str-electropherogram-comparison/main.html"
        width="100%" height="585" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Analyze (L4) — compare STR peak sets across CODIS loci to include or exclude a suspect.

Warm-up. Ask investigators: "A lab does not read your whole genome — it reads a few repeat sites called loci. If a suspect's peaks match the crime scene at four loci but differ at the fifth, are they included or excluded?"

Guided questions:

  • Compare each of the three suspects locus by locus. What does a peak's left-to-right position represent, and what does it mean when two peaks sit at the same position?
  • Why is a single mismatched locus enough to exclude a suspect, while a match at every tested locus only supports inclusion?
  • Switch between Overlay and Side-by-side views. How does each view help you decide whether the orange peaks line up with the blue?

Extension. Research why real CODIS profiles use 20 core loci and how adding loci drives the random-match probability down to one in billions.

References

Specification

This MicroSim was generated from a specification in Chapter 8: Forensic DNA Profiling.

Design note: the five electropherograms are stylized for teaching — peak positions and allele counts are chosen to make locus-by-locus inclusion and exclusion clear, not to reproduce real capillary-electrophoresis traces.