The prosecution theory is: The suspect’s DNA is on the weapon; therefore, the suspect must have handled the weapon. The question for the defense: Is this always true?

The research on DNA transfer says this is not necessarily so. Goray and van Oorshot [1] set up a series of experiments in which volunteers, seated around a table chatted as they handled a pitcher of juice and glasses during a 20-minute social interaction. Prior to the experiment, the pitcher, glasses and other surfaces were treated to remove all DNA and the experiments were videotaped to record who touched which objects. At the end of each experiment, the pitcher, glasses and other objects were DNA profiled to determine the complexity of the DNA mixtures and whose DNA was where. 
The researchers found the following:

1. Quantifiable amounts of DNA were found on most surfaces and items tested.
2. DNA profiles were found on items that were never touched by that person.
3. The DNA was not found on the glass of one (1) participant even though he had handled the glass for majority of the experiment. However, this same participant coughed on his hand prior to handling of the pitcher. He was the major contributor to the three-person DNA mixture on the handle of the pitcher.
4. Approximately 73% of the glasses were DNA mixtures comprised of the holder of the glass along with at least one other person that had not handled the glass.
5. In some experiments, the participants were vectors for DNA from unknown people from the outside that must have been brought into the experiment.
6. While the holder/sitter was the major contributor to his own glass and the surface of the area where he was sitting, there were several samples where the transferred DNA was the major component.
   
   

​So, what does this mean for the trier of facts in the criminal justice system? Clearly, it means that finding a suspect’s DNA on a weapon or surface doesn’t necessarily mean that person handled it. We know that some people transfer more DNA than others when they handle an object [2]. These people are known as good shedders. One reason that an individual may be a good shedder is that they touch their mouths, noses and eyes or they sneeze on their hands, more than poor shedders. Often, the suspects are male, and the weapons are registered to their partners. If the partner handled the gun after sexual contact, the major DNA contributor on the weapon could be the suspect’s. When analyzing a case, one must consider the forensic context: Where was the weapon found? How much DNA was recovered and what was the ratio of male DNA to total human DNA? Was the gun wrapped in a t-shirt? Whatever the defense theory, it needs to be consistent with the forensic evidence.

[1] Goray, M and RAH van Oorschot. 2015. The complexities of DNA transfer during a social setting. Legal Med. 17:82-91

[2] Wickenheiser, RA. 2002. Trace DNA: A review, discussion of theory, and application of the transfer of trace quantities of DNA through skin contact. J Forensic Sci 47(3):442-450