A robbery takes place at the ATM on Campbell Avenue. A passerby who witnessed the robbery was shot. Unfortunately, the security cameras cannot pick up the face of the individual that committed the crime. The camera can identify that the robber left a cigarette butt on the keypad of the machine. There is no evidence of anyone else visiting the ATM between the time the theft occurred and the crime was reported. The camera clearly shows that the suspect left in a service van belonging to a company that provides emergency HVAC repairs. As soon as the crimes are reported and connected, the police collects evidence, including fingerprints on the lock of the machine and the cigarette butt. They also call the HVAC company to find out who was on call around the area where the theft and shooting occurred. The police learns that there were three individuals on call that night servicing the area within 10 miles of the ATM machine. The investigators proceed to obtain DNA from mouth swabs of the three suspects and subject it to short tandem repeat (STR) analysis by comparing it to the DNA isolated from the cigarette butt left at the scene. They use this evidence to determine who committed the crime.
You recall that each person carries two copies of 23 chromosomes (for a total of 46 chromosomes); one copy comes from the mother and the other copy from the father. Each of the 23 chromosomes contains a unique set of genes that code for protein (coding DNA), but a significant portion of DNA contains repetitive sequences that do not code for protein (non-coding DNA). Scientists have figured out that there are certain areas of the genome that contain repetitive sequences that occur in tandem, and the number of times these are repeated vary between individuals. This information can be used to create an individual’s DNA profile or DNA fingerprint. Figure 1 (below) shows specific regions of human chromosomes where the FBI looks for these repetitive sequences (STRs). These regions are called CODIS sites (COmbined DNA Index System).
Figure 2 (below) shows an example of a CODIS region obtained from the DNA isolated from the cigarette’s butt at the crime scene.
You will use this information and visual aids provided to answer the questions on the attached worksheet. When comparing the STRs on CODIS, take into account that as depicted above, the first column corresponds to the sequence of nucleotides on the chromosome inherited from the father and the second column corresponds to the STR sequence inherited from the mother. This extra credit assignment is worth 15 points.
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