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, Ashley Hall and her colleagues 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.
Can you give us a sneak peek into the main theme of your presentation and why it’s important for our audience?
Alternate scenarios to explain why a suspect’s DNA is found at a crime scene are a common form of defense. With the sensitivity of current technologies, we can detect DNA that has been transferred not only during primary contact, but also in a secondary, or even tertiary, transfer. Therefore, it is critical to determine not only whose DNA is present at the crime scene, but also how it got there. Evaluations of the evidence given the donor’s activities inform the “activity-level propositions” that address these questions. We will present a method for describing activities by quantifying DNA transfer and recovery at simulated crime scenes. We describe its application to the assessment of activities in the case of the State of Idaho vs. Brian C. Kohberger.
What inspired you to explore the topic of your presentation?
The collaboration that produced this research began over tapas at an AAFS meeting about a decade ago. It was a partnership between a university lab and a crime lab, which has been critical for our success. We started exploring the dynamics of the DNA contained in fingerprints at a crime scene by developing a fingerprint positive control, and soon realized that we could use the fingerprint control to explore activity-level propositions.
What’s one common misconception about your area of expertise you’d like to clarify?
The DNA contained in a fingerprint is often much greater than trace-level. However, between approximately 50 – 75% of that DNA can be lost through standard collection and analysis.
How do you hope your presentation will impact the audience or industry?
We hope to encourage discussion about addressing activity-level questions at a crime scene, and how this relates to the May 2024 NIST Human Factors recommendations.
Are there any resources or tools you recommend for those interested in learning more about your presentation topic?
There is an excellent book: Forensic DNA Trace Evidence Interpretation – Activity Level Propositions and Likelihood Ratios. By Duncan Taylor, Bas Kokshoorn, ISBN 9781032225814
When you’re not working, what’s your favorite way to unwind or relax?
Walking, spending time with my grandkids and watching murder documentaries. Loudly, if my husband makes me angry.
What’s the best piece of advice you’ve ever received?
It’s not about me.
If you could only eat one dish for the rest of your life, what would it be?
Pizza
What were you doing in 1989, when ISHI first started?
I was graduating high school, celebrating the 80’s with my can of Rave hairspray
Can you share a memorable moment from ISHI that has stayed with you?
A few years ago, I saw a presentation by a first responder on 9/11. His story of what happened that day from his viewpoint and the work that forensic scientists continue to do amazes me.
