Imagine the scenario: your DNA is found on the handle of a knife at a crime scene. If there can be no reasonable dispute that it is your DNA, the critical question becomes – how did the DNA get there? The prosecution hypothesizes that you deposited your DNA on the handle when you used the knife for the stabbing. Your defense counters that you had lunch with the actual perpetrator earlier that day, the two of you shook hands and they transferred your DNA to the knife handle when they did the stabbing. Both transfer scenarios are possible; the question is, which one is more probable?
Alternate scenarios to explain why a suspect’s DNA is found at a crime scene is a common form of defense. With the sensitivity of current DNA technologies, it is not uncommon to detect DNA that has been transferred not only during primary contact, but also in a secondary, or even tertiary, transfer. Evaluating each scenario provides tremendous value when it can be backed by empirical data that aids the trier-of-fact in determining which scenario is most probable.
Evaluations of the evidence given the donor’s activities inform “activity-level propositions,” addressing how the sample got there, rather than simply who it belonged to. Tracking the transfer of touch DNA through a crime scene can provide valuable insight into the activities of one or more individuals if we better understand transfer and recovery. In replicating these activities in controlled experiments to generate empirical data, however, multiple factors can confound quantitative analysis. Chief among them is the high inter- and intra-person variability in the DNA content of a fingerprint, making it difficult to determine starting quantity. Replicating a multi-step transfer pathway also presents problems, as a hand used as a vector in the transfer will donate its own DNA to the experiment.
To control for these variables, we have developed the domesticated fingerprint (DFP) and domesticated hand (Dom-hand), cousins to the naturally occurring wild fingerprint (WFP) and hand. The DFP contains a known quantity of DNA in a background of sebaceous fingerprint chemistry. It is a ground truth sample; its value is known to be true, provided by empirical evidence rather than by inference. The Dom-hand is a nitrile glove to which a mock skin is affixed, a useful surrogate for the human hand that would contribute its own DNA. It standardizes delivery of the DFP and eliminates the human variable. With the domesticated duo, DNA can be tracked across the scene by quantifying DNA recovery at each step of the transfer pathway. The data generated may aid in evaluating the probability of a primary vs secondary DNA transfer.
As a proof-of-concept, we mocked the case of the State of Idaho vs Bryan C. Kohberger. Briefly, on November 23, 2022, four University of Idaho students were stabbed in their off-campus residence in Moscow, Idaho. A knife sheath was found at the crime scene. At the crime lab, DNA consistent with Kohberger’s was found on the snap of the knife sheath. Two of the possible alternate scenarios were: 1) the suspect touched the snap, depositing his DNA – a primary transfer to the snap, or 2) the suspect’s DNA was transferred first to another hand, then to the snap as a secondary contact. We ran the primary and secondary pathways 20X each using domesticated fingerprints and hands. DNA was collected from each surface and recovery was quantified by qPCR. The pathways were repeated an additional 20X each with wild hands and fingerprints for comparison. Welch’s t-test was used to test for differences in the amount of DNA recovered from primary and secondary transfers for both domesticated and wild fingerprints. In both cases significantly more DNA was recovered from primary transfers than from secondary transfers.
We have built a sound and robust model that permits evaluation of different events based on DNA recovery. There is a diminished amount of DNA depending on the number of steps in a sequence, and this sequence can ultimately be evaluated with likelihood ratios. Objective data will permit improved interpretation, and this data will inform interpretation as per the May 2024 NIST Human Factors recommendations.
Imagine the scenario: your DNA is found on the handle of a knife at a crime scene. If there can be no reasonable dispute that it is your DNA, the critical question becomes – how did the DNA get there? The prosecution hypothesizes that you deposited your DNA on the handle when you used the knife for the stabbing. Your defense counters that you had lunch with the actual perpetrator earlier that day, the two of you shook hands and they transferred your DNA to the knife handle when they did the stabbing. Both transfer scenarios are possible; the question is, which one is more probable?
Alternate scenarios to explain why a suspect’s DNA is found at a crime scene is a common form of defense. With the sensitivity of current DNA technologies, it is not uncommon to detect DNA that has been transferred not only during primary contact, but also in a secondary, or even tertiary, transfer. Evaluating each scenario provides tremendous value when it can be backed by empirical data that aids the trier-of-fact in determining which scenario is most probable.
Evaluations of the evidence given the donor’s activities inform “activity-level propositions,” addressing how the sample got there, rather than simply who it belonged to. Tracking the transfer of touch DNA through a crime scene can provide valuable insight into the activities of one or more individuals if we better understand transfer and recovery. In replicating these activities in controlled experiments to generate empirical data, however, multiple factors can confound quantitative analysis. Chief among them is the high inter- and intra-person variability in the DNA content of a fingerprint, making it difficult to determine starting quantity. Replicating a multi-step transfer pathway also presents problems, as a hand used as a vector in the transfer will donate its own DNA to the experiment.
To control for these variables, we have developed the domesticated fingerprint (DFP) and domesticated hand (Dom-hand), cousins to the naturally occurring wild fingerprint (WFP) and hand. The DFP contains a known quantity of DNA in a background of sebaceous fingerprint chemistry. It is a ground truth sample; its value is known to be true, provided by empirical evidence rather than by inference. The Dom-hand is a nitrile glove to which a mock skin is affixed, a useful surrogate for the human hand that would contribute its own DNA. It standardizes delivery of the DFP and eliminates the human variable. With the domesticated duo, DNA can be tracked across the scene by quantifying DNA recovery at each step of the transfer pathway. The data generated may aid in evaluating the probability of a primary vs secondary DNA transfer.
As a proof-of-concept, we mocked the case of the State of Idaho vs Bryan C. Kohberger. Briefly, on November 23, 2022, four University of Idaho students were stabbed in their off-campus residence in Moscow, Idaho. A knife sheath was found at the crime scene. At the crime lab, DNA consistent with Kohberger’s was found on the snap of the knife sheath. Two of the possible alternate scenarios were: 1) the suspect touched the snap, depositing his DNA – a primary transfer to the snap, or 2) the suspect’s DNA was transferred first to another hand, then to the snap as a secondary contact. We ran the primary and secondary pathways 20X each using domesticated fingerprints and hands. DNA was collected from each surface and recovery was quantified by qPCR. The pathways were repeated an additional 20X each with wild hands and fingerprints for comparison. Welch’s t-test was used to test for differences in the amount of DNA recovered from primary and secondary transfers for both domesticated and wild fingerprints. In both cases significantly more DNA was recovered from primary transfers than from secondary transfers.
We have built a sound and robust model that permits evaluation of different events based on DNA recovery. There is a diminished amount of DNA depending on the number of steps in a sequence, and this sequence can ultimately be evaluated with likelihood ratios. Objective data will permit improved interpretation, and this data will inform interpretation as per the May 2024 NIST Human Factors recommendations.
Workshop currently at capacity. A waitlist is available to join on our registration page.
Director, Forensic Science Graduate Program, UC Davis
At UC Davis, Dr. Hall teaches the DNA courses in the Forensic Science Program and runs an active research lab. Her graduate students work on various projects dealing with touch/trace level DNA and how it can be used inform activity-level propositions.
Retired Director for the New York State Police Crime Lab System
Dr. Ray Wickenheiser, is the retired Director of the New York State Police Crime Laboratory System, formerly headquartered in Albany, New York. Ray is now located in Lafayette, Louisiana, where he provides forensic consulting and training. His areas of expertise include crime lab administration, quality management, conflict resolution, forensic DNA and mixture interpretation, serology, hair and fiber trace evidence, physical matching and comparison, glass fracture analysis, forensic grain comparison and forensic investigative genetic genealogy.
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