By Martin Horejsi
Ever since the 1995 O.J. Simpson trial DNA evidence has been a critical and accepted piece of evidence in the courtroom. But even
though DNA evidence is widely used, the mechanics of DNA analysis are not widely understood by the general public.
DNA is essentially a chemical code for an organism with enough minute variations that it is possible to link a specific piece of DNA to a particular individual. But more likely, DNA is used to break the circumstantial link between an individual and a crime proving that the DNA does not belong to a suspect. (Learn about the Innocence Project at http://www.innocenceproject.org).
By using UPC codes which are found on the packaging of everything for sale, many of the facets of DNA analysis can be mimicked in the classroom without the egregious nature for which crime scene DNA is collected. For example, imagine your dog got into a bag of groceries and the only evidence left of what he ate was a portion of the package’s UPC code. Could you identify what the canine consumed?
Progressing from straightforward to more complex, students match a crime scene DNA-UPC barcode with a set of known subject codes. The students soon realize that it is easier to look for markers or pieces of the code that stand out such as two thick lines next to each other or four thin ones. And just as quickly, they realize that their marker allows them to quickly eliminate suspects reducing the number of samples that require closer inspection.
An educational byproduct at this point is that the students appreciate important aspects in the process of science, namely that in hypothesis testing, one needs only a single contrary piece of evidence to force a theory to be modified or abandoned. In the pursuit of a criminal, there is a constant back and forth logic argument until enough of the jigsaw pieces evidence are in place to bridge across all contrary challenges to the conclusion.
To create the pieces for the DNA UPC activity, ultimately the students only need the crime scene DNA and some comparables. By using a photocopy machine, it is possible to amplify the DNA both in size and number. Then compare the DNA to a worksheet full of UPC barcodes. In this basic scenario, there is one crime scene sample and one correct choice on the worksheet of suspects. However, it is easy to modify the activity at this point to increase the difficulty or address other educational objectives. For example, the following changes or additions can be made:
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1.The crime scene DNA can be incomplete such as just a portion of the right edge, the left edge, or a part of the center.
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2.The sample can be damaged to various degrees including water soaking and wrinkling.
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3.The sample can be of odd shape. Similar to an incomplete sample, a non-standard shape, like a scalene triangle or star, may require some extrapolation such as sketching out a larger piece of the code.
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4.Chain of custody issues can be embedded such as how the sample was collected, who had access to it, and if there is any chance the sample could be contaminated. Imagine the students’ hand covered post-it notes while they work with the crime scene sample. Is there any chance that one of the post-it notes could fall off and accidently (or on purpose) wind up in the crime scene evidence bag?
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5.The students can be asked to define the probability that a crime scene sample matches a subject. While the intricacies of the universal product code system could add a fact-based grounding here, it is possible to make educated guesses about how likely it is that a particular portion of code could be found in a complete sample of code. For example, if a code is five centimeters wide, and you have a two-centimeter sample that matches at least one subject, is there a chance that the crime scene sample could match another subject as well? But be aware of the Prosecutor’s Fallacy (click HERE for an explanation) where the probability of multiple DNA matches does not equate to matches across all other aspects of the case.
Once the lab work is done, there is great potential for visuals in a defense or prosecution argument. An authentic assessment for this activity would be to place the student on the (figurative) witness stand to defend their processes and conclusions. And they have to have a conclusion!
Click HERE and you will find a table of 15 UPC codes. Every code belongs to a different individual, and this sheet should be photocopied as a student reference sheet.
For evidence found at the scene you should choose one code for each group of students and cut it out from the reference sheet, and then enlarge or reduce it. You can give the entire UPC code to your students as is, or you give them a portion that is wet, dirty, or smeared by moving it as it is photocopied. Be imaginative! Remember, DNA evidence comes in all conditions, from pristine to unusable. Plus, this approach can be used later in your course if you have a mystery that involves several forensic disciplines combined with DNA.
