DNA Profiling & CODIS: Who Robbed the Bank?

Biomedical engineers who understand the science of genetics create tools, equipment and processes to accurately collect and examine DNA evidence for crime and paternity cases. These engineers also work with attorneys and in court systems to explain how DNA profiling works.

After this activity, students should be able to:

• Describe the organization of DNA into repeating nucleotide base pair sequences
• Explain how DNA profiling is used to link people to crime and paternity cases.
• Describe the role of biomedical engineering in DNA profiling.

Familiarity with DNA and its constituent nucleotide base pairs.

A robbery takes place at a bank. As the thief escapes the building, a security guard grabs one of the bank robber's gloves. The bank robber leaves the scene in a phone service van. The phone company identifies three employees who may have been in the vicinity of the bank at the time of the robbery. All employees deny robbing the bank. Can you think of some way, besides witness testimony, that the bank robber could be identified from among the three individuals?

DNA evidence is more reliable than fingerprints at identifying people.CopyrightCopyright (left) US Geological Survey, US Department of the Interior, http://www.nwhc.usgs.gov/disease_information/avian_influenza/gallery/pages/0025.jsp, (right) President's DNA Initiative, US Department of Justice http://www.dna.gov/training/evidence/

DNA can identify people — even better than fingerprints. DNA is found in all of our cells: hair, teeth, bones, blood and skin. Though all humans share 99.9% of their genes, our DNA differs from everyone else's by three million nucleotide base pairs.

Our DNA is organized in 23 chromosomes in the nucleus in each of our cells. Regions in each chromosome contain what are called "junk DNA," which does not contain genes. But often, this junk DNA contains repeating nucleotide base pair sequences that can be used for matching purposes. (Show students Figure 1 or the same image in the attached CODIS Visual Aid (PDF).) In this example, you can see chromosome locations where the FBI looks for repeating sequences of DNA. They're called CODIS sites, which stands for the FBI's Combined DNA Index System.

Figure 1. The 23 human chromosomes and 13 chromosomal locations at which the FBI looks for repeating DNA sequences. For this activity, note the TPOX region on chromosome 2. (X and Y count as one chromosome pair. The AMELs are not CODIS sites.)CopyrightCopyright Biochemical Science Division, Chemical Science & Technology Laboratory, National Institute of Standards and Technology http://www.cstl.nist.gov/div831/strbase/images/codis.jpg http://www.cstl.nist.gov/div831/strbase/fbicore.htm

In our case, the police found a hair in the bank robber's glove. Remember that we have 23 pairs of chromosomes, each pair containing one chromosome from our father, the other from our mother. A DNA analysis shows that the hair in the robber's glove contains the following nucleotide base pair sequences in the TPOX region (show students Figure 2 or the same image in the attached CODIS Visual Aid (PDF)).

Figure 2. TPOX region of chromosome 2 of hair found in bank robber's glove.CopyrightCopyright TL Program, College of Engineering, University of Colorado at Boulder.

Note that the GAAT sequence is repeated twice in the father's side and three times in the mother's side (the sides of each chromosome are often not the same length). Equivalently, we could say that the CTTA sequence is repeated. Why is that? (G always pairs with C, and A always pairs with T).

So now let's compare the TPOX regions of the DNA found in the bank robber's glove with the TPOX regions of the DNA of two suspects. Note that we are just looking at the one side of the DNA with the GAAT repeating sequence. This simplifies the comparison. (Show students Figure 3 or the same image in the attached CODIS Visual Aid (PDF).)

Figure 3. Comparison of the TPOX region of Chromosome 2 of the unknown bank robber and two suspects.CopyrightCopyright ITL Program, College of Engineering, University of Colorado at Boulder.

Suspect 1 matches the GAATGAAT sequence of the hair found in the glove on one chromosome, but the other chromosome does not match. Both chromosomes must match to show that the hair in the glove came from a specific suspect. Thus, from just one CODIS site we already know that the hair in the bank robber's glove cannot belong to suspect 1.

Suspect 2 matches the GAATGAAT sequence on one chromosome and the GAATGAATGAAT sequence on the other chromosome, so you can say that suspect 2 matches at the TPOX location. To confirm that the hair belongs to suspect 2, the other 12 chromosome locations (see Figure 1) must also match. If all 13 CODIS locations match, then the hair in the bank robber's glove belongs to suspect 2.

The random probability that one of your CODIS sites matches with someone else's is about one in 10 (1/10). Therefore, the probability of two CODIS sites matching is 1/10*1/10 = 1/100 (one in 100). The chance of three CODIS sites matching randomly is 1/10*1/10*1/10 = (1/10)³ = 1/1000 (one in 1,000). The random chance that all 13 CODIS sites match is (1/10)¹³ = one in 10,000,000,000,000. The chance of being struck by lightning in your lifetime is, roughly, one in 1,000,000. So you are 10 million times more likely to be struck by lightning than you are to have the same 13 CODIS sequences as another person. This is what makes DNA profiling so certain.

Engineers can be involved in many aspects of crime scene investigation. They might use their knowledge of CAD (computer-aided drawing) to create a reconstruction of the crime scene. First they might develop a model of the room, and then determine the path of bullets and analyze the blood splatter patterns to determine the position of victims and their killers at the time of the crime. Biomedical engineers create the tools, equipment and processes to accurately collect and examine DNA evidence for crime and paternity cases. Biomedical engineers also help investigators with the analysis of the gene sequencing for DNA profiling.

Background

In this activity, probability is used to determine which suspect is the most likely match. We are told that the likelihood of a random match between a CODIS site for one person and someone else is 1/10. Why is that? Each of the regions that we are considering here contains an allele, or, a version of a gene. Which allele you have at a particular site is determined by your parents' genetic data and meiosis, as well as any errors in replication. However, these alleles are not unique in a population; you can have the same allele as someone else. Statisticians study populations to get an idea about the distribution of alleles (how many people have each kind of allele). In this way, statisticians can estimate a probability that any two people have the same allele. If the likelihood of a CODIS site match between two random people was much greater or much less than the 1/10 used in this activity, the number of matches we would need in order to be reasonably certain of the suspect's guilt would also change.

Before the Activity

• Make copies of the Suspect CODIS Analysis Worksheet (PDF), one per team.
• Set the stage for the activity by conducting the Introduction/Motivation section.

With the Students

1. Divide the class into pairs of students, and pass out a worksheet to each team.
2. Assist students as they complete their worksheets.
3. Have teams conclude by writing on their worksheets which suspect their DNA profiling implicates in the robbery.
4. Have the teams with the correct answer describe how they arrived at their conclusion. (Answer: Suspect 2 seems likely based on a match with four CODIS sites).
5. Have students calculate the likelihood that suspect 2, even though he matches four CODIS sites, is not the owner of the hair in the bank robber's glove. (Answer: (1/10)⁴ = 1 in 10,000, not good enough – need more CODIS site data)
6. Have students act as biomedical engineers and analyze the results of the DNA profiling for the police investigators as described in the post-assessment activity.

Pre-Activity Assessment

Discussion/Brainstorming: As a class, ask students if they can think of some way that a bank robber could be identified if no one saw who he or she was. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Brainstorming is how engineers come up with creative ideas. Have them raise their hands to respond. Record their ideas on the board.

Activity Embedded Assessment

Worksheet: Have students complete the activity worksheet; review their answers to gauge their mastery of the subject.

Post-Activity Assessment

Engineering Analysis: Have students act as biomedical engineers and analyze the results of the DNA profiling for the police investigators. Have each team state which suspect their DNA profiling implicates in the crime. How certain are their results? Next, have the students write a brief one-page report on their results that they might deliver to the police investigators. In this report, they should explain the outcomes of the DNA profiling, how they arrived at their results, and how they determined the certainty of their results.

• Make sure students use care with scissors.

If students have difficulty, work through the first CODIS site on the worksheet with them.

Sometimes it helps to cut out the robbery evidence CODIS data columns from the worksheet and hold them right next to the suspect data columns, making it easier to compare for matches of repeating base pair sequences.

Basic Biology of DNA. About Forensic DNA, President's DNA Initiative, USA.gov. Accessed February 24, 2009. (Glossary is source of some vocabulary definitions, with some adaptation) http://www.dna.gov/basics/biology/

Crime Lab FAQs. Crime Laboratory Division, Missouri State Highway Patrol. Accessed February 24, 2009. (scroll down for DNA analysis and profiling questions/answers) http://www.mshp.dps.mo.gov/MSHPWeb/PatrolDivisions/CLD/faqs.html

DNA Casework. Crime Laboratory Division, Missouri State Highway Patrol. Accessed February 24, 2009. http://www.mshp.dps.missouri.gov/MSHPWeb/PatrolDivisions/CLD/DNACasework/DNACasework.html

DNA Profiling. Crime Laboratory Division, Missouri State Highway Patrol. Accessed February 24, 2009. http://www.mshp.dps.missouri.gov/MSHPWeb/PatrolDivisions/CLD/DNAProfiling/DNAProfiling.html

How DNA Evidence Works. HowStuffWorks. Accessed February 24, 2009. http://science.howstuffworks.com/dna-evidence1.htm

Putting DNA to Work. Marian Koshland Science Museum of the National Academy of Sciences, The National Academies, Washington, DC. Accessed February 24, 2009. (Great DNA reference; topics include DNA and criminal justice; Catch a Criminal activity is inspiration for this activity) https://koshland-science-museum.org/