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miniPCR bio™ Electrophoresis Forensics Lab. Wrongfully Convicted? Instructor's and Student's Guide Version: 1.1 - Release February 2022 - © 2022 by miniPCR bio™ Student's Guide P./13 Remember that STR alleles vary based on the number of repeats, and the DNA from an STR allele with more repeats will be longer than the DNA from an STR allele with fewer repeats (Figure 2). Gel electrophoresis lets scientists visualize these differences in length and differentiate between STR alleles. The gel shown in Figure 6 shows results for a single STR location. We can tell that the evidence DNA sample, suspect 1, and suspect 2 have two different alleles for this STR because the DNA samples show two bands of different sizes. On the other hand, we can tell that suspect 3 has two copies of the same allele for this STR because only one band appears on the gel. When comparing crime scene DNA with DNA from suspects, the unique band patterns can quickly eliminate suspects whose STR profile does not match the evidence. For instance, in the gel shown in Figure 6, the DNA fragments of suspect 1 and suspect 3 do not match the evidence, so these suspects can be excluded as being the source of the DNA evidence. On the other hand, suspect 2's DNA bands do match the evidence, which means that suspect 2 cannot be excluded as being the source of the DNA evidence. It is crucial to keep in mind that forensic investigations typically examine at least 20 STR regions, Figure 6. Interpreting gel electrophoresis results This gel shows results examining a single STR location in the genome. By comparing the pattern of bands from the DNA evidence to the DNA of the suspects, forensic scientists can rule out suspects 1 and 3. However, suspect 2 matches the DNA evidence and cannot be excluded from further investigation. DIRECTION OF MIGRATION BASE PAIRS 400 300 200 100 LADDER EVIDENCE SUSPECT 1 SUSPECT 2 SUSPECT 3 and even if someone's complete STR profile matches the DNA evidence, it does not prove their guilt. If forensic scientists get a DNA match, then the last step of forensic DNA analysis is to calculate how common that particular STR profile is (Figure 5). The random match probability represents the chance that two unrelated individuals share the same STR profile. It takes into account the number of STR regions compared, the number of different alleles for each STR location, and how common each allele is in the general population. Modern STR analysis can yield genetic profiles that are statistically quite rare, on the order of one in many trillions. For more information on the statistics related to STR profiles, refer to the extension activity on probability and DNA profiles (page 25). Remember though, that even if the statistical analysis very strongly suggests a DNA match, that still doesn't prove a suspect's guilt. It only supports that the suspect's DNA was present.

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