Guns, Gams & Gumshoes

A defense attorney & PI who also happen to be writers

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Answering Writers’ Questions: Do DNA Samples Have to Be Perfect?

Posted by Writing PIs on November 12, 2010

FDA microbiologist prepares DNA sample (image is in pub domain)

Today we’re posting some writers’ questions, and our answers, about DNA samples.

Writer’s Question: Does a DNA sample taken at a crime scene have to be perfect?  Does it have to be intact and isolated?  What if a DNA sample is picked up by a sleuth long after the police have left the crime scene—the DNA is from the floor of an auto garage where there’s oil, gas, etc. as well as dried blood.

Guns, Gams, and Gumshoes’ Answer: PCR (Polymerase Chain Reaction) technology makes it possible to isolate identifiable strands of DNA, even when the sample is mixed with other chemicals and organic substances.  For instance, scientists in Australia were able to recently extract and identify blood in a leech as containing the DNA of an identifiable human suspect who has since been convicted of robbing someone (and having left a leech at the crime scene).  In this example, PCR technology was able to differentiate between the human and animal chromosones.

Writer’s Question: Can one private forensics lab screw up DNA results, while another provides better, more realistic results?

Guns, Gams, and Gumshoes’ Answe: Absolutely.  There are certain labs that have had their results reviewed by a second opinion (from an outside lab) and it has been shown that the original lab used improper techniques or slanted calculation statistics (as to the population).  How might a sleuth research if a private forensics lab is subject to criticism?  One way is for a sleuth (or a fictional sleuth in a story) to go to an organization that would have expert feedback on forensic science third-party experts (from qualified private forensic labs to fingerprint experts to polygraph examiners and more). Examples of such organizations are the state/national criminal defense organizations, the state/federal public defenders offices, state/national trial lawyers organizations.  As a writer, if you (or your sleuth in a story) were wanting a recommendation for a qualified forensic lab in a particular state, you/the fictional sleuth might google one of these organizations, then write a letter to their director (who, if he/she can’t help you, will most likely route it to a contact who can).

Last note:  Some of the organizations we mentioned maintain databases of such experts.  In more than a few instances, individual attorneys coming up against a lab in a court case have used their own investigators to research and determine the pros/cons of a particular lab.  This data is then added to the database.   Faults these investigators might learn about labs are how they inadequately store evidence samples, how scientists might be overworked and prone to making mistakes, or if a lab has financial problems (which causes them to cut corners, which in turn would most likely affect the integrity of their results).

Have a great week, Writing PIs

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