DNA's long-term stability and its ability to precisely confirm the identity of a potential suspect in criminal cases have supported its continuous use for forensic analyses.
Recent advancements in molecular biology techniques have allowed scientists to determine an individual’s ‘DNA shedding’ status, which can improve forensic investigations and, more precisely, determine the role of an individual's presence during a crime.
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What is DNA Shedding, and Why Does It Matter in Forensics?
DNA is present in most of the cells in our body, which is unique in each and every individual, and we leave a trail of it everywhere we go.1”
Previously, forensics studies assumed that any DNA recovered from touched items or surfaces originated from epidermal cells. However, recent studies have demonstrated that both nucleated cells and cell-free DNA can be transferred onto items and surfaces from saliva, sweat, and sebum.
These discoveries led to the concept of ‘shedder status,’ which refers to the tendency of an individual to ‘shed’ their DNA onto an item or surface following contact.2 Existing evidence suggests that some individuals can be classified as ‘good shedders,’ as they consistently deposit more DNA than ‘poor shedders.’
In addition to individual variability, other factors that may impact the quantity of DNA shed by an individual include gender, as men generally shed more DNA than women, age, different events, behaviors, environments, and types of activity.
Uses of Microscopy in Forensics
How DNA Shedding is Measured
Determining an individual’s shedder status can provide important insights into any forensic investigation; however, the only way to currently determine this characteristic is through a reproducible, robust, and easy-to-perform test. The first report of DNA shedding involved having individuals hold a test tube for 15 seconds with their whole hand, following which swabs are applied to the tube where contact was made. Researchers have also investigated how the placement of an individual’s hand on a glass plate for 15 seconds can also be used as a platform to investigate their shedder status.3
Regardless of whether DNA shedding is being measured during a similar test or forensic investigation, samples are carefully collected and stored based on the type of sample. For example, whereas blood samples will be stored at -20 °C or -80 °C for several weeks or longer, respectively, epithelial cell samples are maintained in a dry environment and stored at room temperature.1
Typically, DNA analysis will begin with an extraction phase, during which cells are lysed, membrane lipids are removed, protein is denatured, and all other cellular components and ribonucleic acid (RNA) material are removed. Thereafter, DNA is quantified and subsequently amplified through a polymerase chain reaction (PCR) assay. For forensic applications, DNA-amplified products can be detected through several methods, some of which include autosomal short-tandem repeat (STR) profiling, mitochondria DNA (mt-DNA) or Y chromosome analysis, or autosomal single-nucleotide polymorphism (SNP) typing.1
To reduce the high costs associated with these laboratory analyses, cell staining, and counting methods have also been investigated for their ability to capture DNA shedding. Cell staining often involves the application of a nucleic acid dye on sample slides to visualize the presence of DNA in cells; however, this method is associated with several limitations. When used to analyze skin cell samples, keratinocytes and corneocytes consist of varying amounts of chromosomal DNA, including zero, which can negatively impact the accuracy of DNA quantification in cell staining.3
Cell counting, a similarly low-cost method, can provide results immediately using a mini microscope with fluorescence capabilities. Nevertheless, this technique is also associated with certain limitations, as it cannot distinguish DNA-free cells from those containing DNA.
In the event there is a low amount of DNA present in the sample or DNA degradation occurs, more advanced techniques may be needed to handle these challenging samples. Next-generation sequencing (NGS) technologies, for example, allow for the high-throughput sequencing of samples, even in samples with low or degraded DNA.5
How has DNA Analysis Evolved in Forensics?
Distinguishing Primary and Secondary Transfer
In criminal cases, DNA can be transferred from primary and secondary contributors. Some examples of primary DNA transfer include blood drops from an individual after being injured, semen ejaculated onto a surface or individual, directly touching a surface or another person with bare hands, as well as clothing or jewelry that was in direct contact with an individual’s skin.6
Secondary or indirect DNA contributors refer to unrelated individuals who indirectly transfer their genetic material through certain objects or individuals. For example, if an individual shakes hands with a second person, who subsequently touches a knife obtained at a crime scene, the DNA of the first individual may be transferred to the knife as well.4
Various factors can contribute to how much DNA is transferred during these events, such as the individual’s shedder status, further demonstrating the crucial role that DNA shedding can have in forensic investigations.
The Future of DNA Shedding Analysis in Forensic Science
Artificial intelligence (AI) technologies like machine learning (ML) have been applied to almost every industry; however, their potential role in forensic analyses remains largely unexplored. Nevertheless, researchers predict that ML methods can support DNA analyses by generating genomic annotations, predicting functional genomic elements, and clarifying different mechanisms of gene expression.6
Within forensics, ML can support decision-making processes in an unbiased manner while also analyzing a substantial amount of data. While promising, additional research is needed to create large training sets for ML systems while remaining transparent for legal purposes.
Conclusions
Despite extensive studies that have aimed to clarify DNA shedder status and its role in criminal investigations, additional research is needed to develop standard methods to characterize individuals as poor, intermediate, or good DNA shedders.
In fact, recent studies suggest the existence of five shedder types including light, intermediate-light, intermediate, heavy-intermediate, and heavy shedders.
An individual’s shedder propensity sits on a continuum, rather than in ‘bins’ of low and high or even low, medium, and high.3”
Investigating the Internet of Things Role in Forensic Investigations
References
- Bukyya, J. L., Tejasvi, M. L. A., Avinash, A., et al. (2021). DNA Profiling in Forensic Science: A Review. Global Medical Genetics 8(4); 135-143. doi:10.1055/s-0041-1728689.
- Jansson, L., Swensson, M., Gifvars, E., et al. (2022). Individual shedder status and the origin of touch DNA. Forensic Science International: Genetics 56. doi:10.1016/j.fsigen.2021.102626.
- Petcharoen, P., Nolan, M., Kirkbride, K. P., & Linacre, A. (2024). Shedding more light on shedders. Forensic Science International: Genetics 72. doi:10.1016/j.fsigen.2024.103065.
- Sessa, F., Pomara, C., Esposito, M., et al. (2023). Indirect DNA Transfer and Forensic Implications: A Literature Review. Genes 14(12). doi:10.3390/genes14122153.
- Haddrill, P. R. (2021). Developments in forensic DNA analysis. Emerging Topics in Life Sciences 5(3); 381-393. doi:10.1042/ETLS20200304.
- Barash, M., McNevin, D., Fedorenko, V., & Giverts, P. (2024). Machine learning applications in forensic DNA profiling: A critical review. Forensic Science International: Genetics 69. doi:10.1016/j.fsigen.2023.102994.
Further Reading