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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™ Additional Supports P./27 Interpreting random match probabilities Multiplying the genotype frequencies for all 20 STR locations used in CODIS using the product rule gives random match probabilities that can be quite small, like one in many trillions. However, there are instances where the random match probability can be much larger. For example, if someone has several STR genotypes that are more common in the general population, then the chance of a random match will be more likely, though typically still very rare. Further, sometimes DNA evidence degrades before it is collected. In these cases, it might not be possible to analyze as many STRs. And with fewer STRs, the chance of getting a random match becomes more likely. It is essential to note that the random match probability only represents the rarity of a specific genetic profile. Unfortunately, random match probabilities have been misinterpreted in many legal settings. For example, given a random match probability of 1 in 5 trillion, a prosecutor might claim that there is only "a 1 in 5 trillion chance that the defendant is innocent." This error in reasoning is referred to as the prosecutor's fallacy. It ignores additional factors that might make matching the DNA profile at the crime scene more or less likely. For example, the person may have been at the crime scene for reasons unrelated to the crime. Naming of STR locations Before, we referred to STR locations with simplified names like STR 1 and STR 2. In reality, forensic scientists use a complex naming system for regions of the genome with STRs. In the following questions, we will use the actual STR names because you will be using real data on the frequencies of the STR genotypes in the human population. Most of the STR locations used in forensic analysis are in the regions of the genome that do not contain genes. These STR regions are named using a standardized system (Figure 2). Each STR name starts with a D for "DNA," followed by a Figure 2. STR naming conventions D7S820 unique identifier chromosome number number that tells you which chromosome this STR is on. Then there is an "S" followed by another number. This second number is needed because there are many different STR regions on each chromosome, so each one needs to have a unique identifier.

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