miniPCR bio™ Electrophoresis Forensics Lab. Wrongfully Convicted? Instructor's and Student's Guide
Version: 1.1 - Release February 2022 - © 2022 by miniPCR bio™
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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.
