Activity 1.2 3 Suspect DNA Answer Key: Exact Answer & Steps

So You’re Stuck on Activity 1.2.3 “Suspect DNA” — You’re Not Alone

Let’s be real for a second. You’re here because you’ve got a worksheet in front of you titled something like “Activity 1.So 2. 3 Suspect DNA” and you have no idea what’s going on. Maybe it’s a genetics assignment, a forensics simulation, or part of some biology module. You’ve stared at the gel electrophoresis image, the DNA fragment sizes, and the suspect profiles, and your brain just… stalled.

Not obvious, but once you see it — you'll see it everywhere And that's really what it comes down to..

Take a breath. Also, most textbooks and handouts give you the steps but skip the why. That said, it’s about a poorly explained activity that assumes you already know how DNA fingerprinting actually works in practice. On top of that, this isn’t about you being bad at science. So you’re left matching numbers, hoping something sticks It's one of those things that adds up..

That’s where this guide comes in. In practice, we’re going to walk through what this activity is really asking, how to think about it, and—yes—what the answer key would show. But more importantly, you’ll understand why it’s the answer, so the next time something like this pops up, you’ll know what to do. No more guessing And that's really what it comes down to..

What Is Activity 1.2.3 “Suspect DNA” Really?

First, let’s clear up what this activity is. It’s almost certainly a simulated DNA fingerprinting exercise. DNA fingerprinting—also called DNA profiling—is a technique used in forensics to identify individuals based on unique patterns in their DNA. STRs are sequences where a short pattern of DNA bases (like ATCT) repeats a different number of times in different people. The core idea is that while most of our DNA is identical to other humans, certain regions are highly variable. These regions are called Short Tandem Repeats, or STRs. The number of repeats at several STR locations creates a unique barcode for each person—except identical twins Nothing fancy..

In a classroom activity, you’re usually given:

• A crime scene DNA sample.
• DNA samples from several suspects.
• A table or image showing the sizes (in base pairs) of DNA fragments from each person at several STR locations.
• The question: Which suspect’s DNA matches the crime scene sample?

Sometimes it’s presented as a gel electrophoresis image, where fragments are separated by size and show up as bands. Your job is to compare the band pattern from the crime scene to the suspects’ band patterns.

The activity is trying to teach you how DNA profiling works in a simplified way. But without a clear explanation of how STR analysis translates to fragment sizes, it can feel like random number matching.

Why This Activity Matters (And Why It Trips Everyone Up)

Here’s why teachers assign this: DNA profiling is a real, powerful tool used in criminal investigations, paternity tests, and identifying remains. Understanding the logic behind it matters—not just for a test, but for being an informed citizen. When you hear about DNA evidence in a news story, you should know what they’re actually talking about.

The problem is that most activities like this get two things wrong:

1. They assume you know how PCR and gel electrophoresis create the data. In reality, DNA profiling involves extracting DNA, using PCR (Polymerase Chain Reaction) to amplify specific STR regions, and then running those fragments through a gel or capillary sequencer to measure their sizes. The numbers you see are the measured lengths of those amplified fragments Small thing, real impact. And it works..

2. They treat it like a simple matching game. But it’s not just “find the same numbers.” You have to understand that each STR location has two alleles—one from each parent. So a person’s profile at one STR location is usually two numbers (e.g., 120 and 140 base pairs). The crime scene sample also has two numbers at each location. A match means all the numbers line up Simple, but easy to overlook..

When students miss this, they either guess randomly or match one or two numbers and think that’s enough. That’s a recipe for getting it wrong on a quiz—or misunderstanding how DNA evidence actually works.

How DNA Profiling Works in This Activity (The Step-by-Step Logic)

Let’s break down the process so you can actually do the activity instead of just hoping.

Step 1: Understand the Data Table or Gel

You’ll get something like this:

Each row is a different STR location. Each column (except the first) is a person’s DNA profile at those locations. The numbers are the sizes of the DNA fragments, measured in base pairs.

If it’s a gel image, the bands represent those fragments. The crime scene lane will have a certain pattern of bands; you compare it to each suspect’s lane Worth keeping that in mind..

Step 2: Compare One Location at a Time

Start with the first STR (TH01). Look at the crime scene: it has two numbers, 180 and 190. Now look at Suspect A: 180 and 190. That’s a match. Suspect B has 185 and 190—one matches (190), but the other doesn’t (185 ≠ 180). So Suspect B is out. Suspect C has 180 and 195—one matches (180), but 195 ≠ 190. So Suspect C is out Worth knowing..

Step 3: Check All Locations

If a suspect matches at every STR location, that’s strong evidence they could be the source. If they differ at any location, they are excluded. In our example, Suspect A matches at TH01, vWA, and FGA. Suspects B and C fail at least one location.

Step 4: Consider the “Why” Behind the Numbers

Remember, these numbers come from the actual number of repeats in the STR region. More repeats = longer fragment. The PCR primers bind on either side of the STR, so they amplify the whole repeat region plus a little flanking DNA. That’

The gel itself is the visual record of those amplified fragments. Which means smaller fragments travel farther than larger ones, so each band corresponds to a specific length of DNA. Consider this: after the PCR reaction finishes, the mixture is loaded onto an agarose matrix and an electric current is applied. By aligning the lanes side‑by‑side, you can directly compare the pattern of bands in the crime‑scene lane with those of each suspect. If a suspect’s lane contains exactly the same pair of bands at a given locus as the crime scene, the two alleles are considered a match; any discrepancy—whether a missing band, an extra band, or a shift in position—excludes that individual from further consideration Not complicated — just consistent..

Because the STR loci are amplified independently, the analysis proceeds locus by locus. On the flip side, a full profile is built by recording the pair of fragment lengths for every locus examined. Only when all of the loci show identical pairs does the profile be deemed a match. This “all‑or‑nothing” criterion reflects the fact that the probability of two unrelated individuals coincidentally sharing the same combination of alleles across multiple loci is astronomically low. In practice, forensic analysts assign a random‑match probability (RMP) based on the population frequencies of each allele; the more loci that agree, the smaller the RMP, and the stronger the evidential weight That alone is useful..

When the activity includes a gel image rather than a simple data table, students must first interpret the banding pattern. So they locate the band positions by comparing them to a DNA ladder that contains fragments of known sizes. Think about it: by estimating the size of each unknown band relative to the ladder, they can assign the appropriate base‑pair values to the alleles. This step reinforces the concept that the numbers are not arbitrary labels but measurable physical dimensions of the DNA molecules.

Beyond the basic matching exercise, it is valuable to discuss the limitations inherent in the method. Contamination, amplification bias, or degraded samples can produce ambiguous or misleading band patterns. Here's the thing — mixed samples—where DNA from several contributors is present—complicate the interpretation because multiple alleles may appear at a single locus, requiring careful deconvolution. Worth adding, the statistical interpretation of a match must consider the population genetics of the alleles in question; a rare allele combination can dramatically affect the calculated RMP.

It sounds simple, but the gap is usually here.

Understanding these nuances prepares learners for real‑world forensic work, where the visual evidence is only one component of a larger evidentiary picture that includes laboratory quality controls, chain‑of‑custody documentation, and expert testimony Small thing, real impact. No workaround needed..

In a nutshell, the activity demonstrates how PCR amplifies specific regions of the genome, how electrophoresis separates the resulting fragments by size, and how careful comparison of allele profiles at multiple STR loci can either implicate or exclude a suspect. By recognizing that a true match requires agreement at every locus and appreciating the statistical and technical caveats involved, participants gain a realistic appreciation for the power—and the responsibility—of DNA profiling in criminal investigations.