In criminal investigations, DNA analysis plays a central role in identifying the perpetrators of crimes and offenses. However, not all biological traces provide the same type of information. Touch DNA—deposited involuntarily on a surface after simple contact—remains challenging to interpret for forensic experts.
Why do some individuals leave more DNA than others? A recent study conducted by researchers at Flinders University in Australia proposes an innovative method to objectively assess this variability. By examining the individual propensity to shed skin cells, the team opens new perspectives in forensic genetics and the interpretation of biological traces at crime scenes.
A genuine interindividual variability
Some individuals, described as “good shedders,” naturally deposit large quantities of skin cells on objects they handle. Others, by contrast, leave only minimal traces. This difference, long observed by forensic biologists, complicates the interpretation of DNA results, particularly when assessing the likelihood of direct contact between a person and an object.
Until now, reliably and reproducibly quantifying this variability has been difficult. The Australian study specifically addresses this gap, providing a rigorous scientific protocol.
A simple and reproducible measurement protocol
The researchers developed a protocol based on a series of controlled contacts carried out by 100 participants, each asked to touch a standardized surface. The deposited cells were then:
- Stained with a fluorescent marker,
- Counted using microscopy,
- Subjected to genetic analysis to confirm the presence of recoverable DNA.
The results showed that, for 98 out of 100 participants, the level of cell deposition was stable and reproducible over time. This protocol allows individuals to be classified into three categories: high, moderate, or low skin cell shedders.
A tool to better contextualize touch DNA evidence
The value of this method extends beyond biology. It may serve as a tool for judicial contextualization. For instance, a suspect identified as a high shedder could account for the abundant presence of their DNA on an object without having taken part in the offense. Conversely, the absence of DNA from a low shedder does not exclude the possibility of contact.
This information could be incorporated into likelihood ratio calculations used in DNA interpretation, thereby strengthening the robustness of forensic assessments.
Future perspectives for forensic science
The proposed method has several advantages: it is inexpensive, easy to implement in the laboratory, and could be adapted to various objects and realistic conditions (different surfaces, durations of contact, humidity). Further validation studies are still required before widespread adoption. Ultimately, however, this approach could be integrated into routine biological trace analysis, providing valuable support to magistrates and investigators in evaluating the probative value of DNA evidence.
References
- Petcharoen P., Nolan M., Kirkbride K.P., Linacre A. (2024). Shedding more light on shedders. Forensic Science International: Genetics, 72, 103065, read here.
- Flinders University. (2024, August 22). Heavy skin shedders revealed: New forensic DNA test could boost crime scene investigations. ScienceDaily, read here.
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