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Sunday, March 3, 2024

Column: Real life forensic science

Popular TV shows or movies are sometimes difficult for me to watch. For example, the show “Heroes” sounded interesting at first, but I had to stop when halfway through the first episode a college professor repeated the “you only use 10 percent of your brain” myth.

It’s not just science fiction shows, though. I cringe every time a show or movie depicts an investigator at a crime scene picking things up with his bare hands. If the scene were set decades in the past I can understand people being less picky than they are now and the scene wouldn’t bother me. However, the movie may be set in present day, but the investigators don’t give a damn about contamination of the scene.

High budget movies don’t usually make such obvious mistakes anymore. Low budget or made-for-TV movies, however, are guilty of these kinds of things all the time. Bullet casings? Pick them up off the ground without finding out if their location has been noted or photographed. Blood? Smear it between your fingertips (I guess to make sure it isn’t actually cherry Kool-Aid).

Despite the flak that it gets for how it depicts police procedure, which is exciting for everyone, the TV show “CSI” isn’t too bad. I realize that a certain amount of realism has to be sacrificed in order to tell stories to a large audience within a certain time constraint.

However, watching these shows can give a rather skewed view of just how specific forensic science can be and that not every case that is presented to a forensic scientist will be about murder. We all know about forensic DNA analysis, but what about forensic astronomy?

Forensic astronomy isn’t really used much in murder trials but has been used several times by art historians. Vincent van Gogh’s painting “Evening Landscape with Rising Moon” was dated to 9:08 p.m. on July 13, 1889 by determining the exact spot from which van Gogh viewed the moon and at what time and date the moon was in that position.

Crime TV shows generally don’t focus on art history mysteries. The topics of most popular shows revolve around murder or attempted murder. Despite the fact that hardly any actual murder cases have physical evidence at the scene, the TV audience is going to want to see blood and see improbably attractive people in white coats analyze it.

How do they do it? The technical method of analyzing DNA is complicated; just look up DNA profiling on Wikipedia and try not to become dizzy at all the acronyms they use. The very basic idea is that DNA is like an alphabet that contains four letters that can then be arranged into more complicated instructions. The full set of DNA in a person, called the genome, contains a lot of segments of repetitive information. We all have these repetitive segments, but the exact sequence and number of these segments is different for everyone.

Once the scientists have the sample of suspect DNA, perhaps from the crime scene, they can get to work. They use a polymerase chain reaction (PCR) to make many copies of the DNA they want to analyze and then promptly chop up the DNA into very precise but small pieces, cutting only when a certain sequence occurs in the chain.

The DNA pieces are injected into an electrically charged gel that separates the pieces by size. By observing how many pieces of each size are in the sample, scientists can figure out which repetitive sequences are in the sample and how many.

By doing this technique in several different areas of the genome, the probability that two different people would have the same DNA profile decreases. This is why DNA evidence presented in court sounds so convincing; when the odds that an innocent person happens to have the same profile go to one in quintillion (which is one with 18 zeroes after it), a jury member probably wouldn’t think that an accidental match could happen.

In practice, these probabilities can become muddied. Despite the long odds, false matches can happen, and do so much more frequently than expected. Lab contamination can also confuse the results.

Typically labs must use multiple samples and tests to ensure that their results are accurate. This is easy in a civil case where someone is trying to establish paternity but is much more difficult at a crime scene where investigators are lucky to find any physical evidence from the suspect at all.

While watching “CSI” and similar shows, remember that not every case has or needs physical evidence at the scene in order for the mystery to be solved. Old-fashioned detective work is what usually solves the crime, with a helpful assist from physical evidence if it is there.

AMY STEWART can be reached at science@theaggie.org.


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