Given the continuing popularity of “CSI” and similar TV shows and movies, it’s not surprising that juries expect prosecutors to present DNA evidence for every case they bring to court. The reality, of course, is not that simple. In the 1980s, in order to perform DNA analysis on a crime scene or a victim, forensic investigators needed a blood sample or semen stain of about the size of a quarter. The sample size fell in the early 1990s to the size of a dime; by the end of the ’90s it became even smaller: If it is visible it can be analyzed. Recently, however, scientists have developed a new technique for processing DNA called “touch DNA.” With this technique, scientists can test for DNA without a sample from blood or bodily fluids. As we’ll see, this advance offers many advantages and opens up new opportunities for solving cases.
As the name implies, touch DNA is used to process an item someone has touched. Because we are constantly shedding skin cells, when we touch something, we leave skin cells behind. Even if suspects leave only 6-8 skin cells from the outermost layer of their skin, scientists can use those cells to develop a DNA profile. With Polymerase Chain Reaction (PCR) analysis, scientists can take a small sample of skin cells and make as many copies of the DNA as they need to perform their analysis. This process provides a highly specific genetic portrait of the person profiled. And the whole process only takes a few days—much less time than is needed for traditional DNA processing.
Of course, in order to take advantage of touch DNA, you have to collect the right samples. This technique can be used on samples taken from guns, steering wheels, cell phones, glass, plastic, wood, cloth, fabric, etc. While you might be tempted to take samples from every single surface and object at a crime scene, you have to be smart about what you’re doing. Focus on the places a suspect is likely to have touched. If you’re processing a vehicle, you don’t need to take samples of the entire dashboard, for example. Instead, process the steering wheel; the door and the door handles; the rear view mirror; the shifter; the controls for the windows, the stereo, the air, etc. When your crime scene is indoors, observe the scene. Did the suspect try to cover up by washing his hands? Take samples from the faucet and sink surfaces. Look for bathroom or kitchen towels or discarded paper towels. Also remember to process the doors and windows that a suspect may have used to enter or exit. Sample any clothing that may belong to the suspect as well. Finally, look for any items that are out of place—chances are, the suspect was the one who moved them.
When you take your samples you can either use the cut method or the swab method. The cut method allows you to take samples from soft surfaces like cloth, fabric, etc. This method works well for items like shirt collars where there is likely to be sweat and skin cells. The swab method is used on hard surfaces like glass, plastic, wood, etc. As you can see from the photos, you can use this method to swab the trigger, hammer, or grip of a gun or the surface of a cell phone. Getting DNA evidence from guns and cell phones can be crucial to your case, since it can be very difficult to get fingerprints from these objects.