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DNA Standards Often Make the Difference Between Life and Death

by Michael Fortino, Ph.D.

DNA analysis standards can make the difference between misleading results and the truth behind a crime, so much so, that this highly sensitive analysis is often the deciding factor in ruling life or death for many capital murder suspects. Obviously, science needs to get it right the first time because there may not be a second chance.

Nancy Dinh, a forensic DNA analyst at Forensic Analytical Crime Lab in California, presented in early 2021 at the February gathering of the American Academy for Forensic Science (“AAFS”) where she highlighted the significance of DNA analysis and new advancements within the field that move law enforcement further away from theory and closer to fact.

Dinh primarily works on behalf of criminal defendants by “assisting attorneys in understanding results from outside laboratories.” In other words, she is able to guide attorneys through complex DNA analysis and provide a layman’s perspective and an approach to presenting the material in a format that is easier for a jury to comprehend.

Dinh’s work also involves testing various crime scene samples and analyzing evidence to determine the strength and reliability of an alternate or conflicting finding to that of the prosecution.

In her presentation at AAFS, Dinh shared some interesting case examples that teach valuable lessons for other analysts within the field and for individuals hoping to better understand the use of DNA analysis in criminal prosecution.

The first case she presented was a 2015 sexual assault of a minor where the grandfather was named as the initial suspect. The previous lab had made cuttings from small portions of the minor’s undergarment in order to test for DNA. Only a miniscule trace of male DNA was found within the cuttings, and a “Y” chromosome test narrowed potential donors to all the males within the victim’s family. The grandfather, who had initially denied the allegations, changed his story and said his son, the girl’s father, likely committed the assault.

Dinh processed the garment sample utilizing a different analysis, one in which she focused not on the cutting, but rather on a microscopic stain in a different area which provided enough DNA from semen to accurately identify the suspect. The stain had been missed by the previous lab, and “subsequent autosomal testing revealed the grandfather as the match.” Dinh’s lesson here is that flawed or limited sampling can “miss crucial parts and ultimately deliver incomplete and maybe even a false narrative behind the evidence.”

Her second case study involved the 1986 homicide and sexual assault of a female victim. A sexual assault analysis was administered; however, it was not processed until 2003—17 years later. The original testing, which included limited body fluid swabs, proved inconclusive. The kit, using older methodology, was retested again in 2004 and 2016 with no better results.

When Dinh processed the kit in 2020, there was not much forensic evidence remaining to analyze. She explained, “I opened it up and all that remained was a nose swab and some empty vials—one of which was labeled anal swab.” She noticed some faint residue in the vial, so she proceeded to resample this microscopic material. Testing showed that the sample did, in fact, contain at least two male DNA readings. At the time of her presentation in February 2021, a CODIS search was pending on the profiles, and it seemed likely that the analysis would result in a general identification. Dinh assured her audience that the takeaway lesson is, “[j]ust because a parent specimen or substrate is not there, all hope is not lost.”

The third case she presented involved two guns recovered by police in 2018 after multiple suspects shot a man in a vehicle. One suspect was identified from an analysis done on the first gun, but the previous lab’s testing of the second gun was only able to exclude two suspects. Dinh obtained the second gun for retesting in 2020.

Dinh explained, “the previous lab did a general swab of the entire gun, which is fine in theory if you’re just looking for biological [sic] from the gun in general, but, in my experience, gun samples always result in heinous mixtures. So my preference is to sample different parts of the gun separately to minimize the chance of getting complicated or uninterpretable mixture DNA profiles.”

Dinh’s approach, though not typical, led to a breakthrough. In this particular case, and on this particular weapon, she found “semen on the trigger” resulting in a clear profile of a man who “was known to the police but was not a suspect.”

Lesson number three, she comments, “Use past experiences with similar items to guide your testing … even though microscopy isn’t typical, it’s still a powerful tool. It’s time consuming but you never know what you may find.” Obviously, no one in this case was expecting to find semen on the trigger of a murder weapon.

Dinh’s ultimate lesson for DNA analysts and defense attorneys is this: a previous analysis is not conclusive, and subsequent testing may “find the exact opposite results.”

“Automation has overtaken our laboratories,” lamented Dinh. “As a result, the quality of work in some cases could be compromised, or an interesting detail could be overlooked simply because our end goal seems to have shifted from finding truth in evidence to doing just enough to make someone happy—ultimately, the goal is to find truth in DNA evidence.”

Anything less could end the life of an innocent suspect, and a false analysis could free a murderer to kill another day. 



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