CRIMINAL OR SECONDARY TRANSFER?

As a rookie forensic scientist, you were sent on your first duty to investigate a murder case. It appeared that the victim had choke marks on their neck. You thought to yourself: if a person is being strangled, there is a high chance that they tried to fight back, using their hands to attack, which must have left “Touch DNA” -genetic material that’s left when a person comes in contact with a surface- on them. In a rush, you collected a swab sample from their hands and sent it to the laboratory for further investigation. With the modern equipment that had developed to be more sensitive and precise, the DNA (Deoxyribonucleic acid) analysts concluded that a significant portion of the epithelial cells you had sent contained the genetic material of the victim’s sister. “Mission accomplished. The DNA will be submitted as evidence, and the sister will be prosecuted. And just like that, justice is served.” You said to yourself with confidence. Little did you know, you fell right into the trap of secondary transfer DNA. The phenomenon known as “Secondary transfer DNA” occurs when a person’s genetic material is transferred to an object or to another person through an intermediate, meaning it requires no direct contact between the source and the final surface. The quantity of the transferred genetic material could depend on factors such as contact type, moisture, substrate, and shedder status. Studies have shown that high friction, moisture, non-porous surfaces, and good shedders provided the best conditions for successful DNA deposit. The highly sensitive DNA profiling technology can make secondary transfer DNA more detectable. This can lead to false prosecution or suspicion, since scientists struggle with differentiating primary transfer DNA, direct deposition through contact, and secondary transfer DNA. Especially in recent years, there have been cases in which innocent people have been wrongfully convicted of various crimes, including burglary or even murder. One of the cases where DNA implicated an innocent person via secondary transfer could be exemplified by the Lukis Anderson case. An unhoused man in California, Lukis Anderson’s genetic material was found under the fingernails of the Silicon Valley multimillionaire Raveesh Kumra, who was murdered in November 2012. Due to the biological evidence, he was charged with murder, and faced the possibility of a death sentence, despite having no memory of the incident. It was later revealed by the legal team that Anderson’s DNA found its way to the crime scene via the paramedics who arrived at Kumra’s mansion. Anderson had been treated by the same paramedics that day, which caused his DNA to be carried to the crime scene and led him to be implicated unjustly, although charges were dropped due to his alibi of being hospitalized and unconscious during that time being confirmed. However, the 5 months he had lost in jail due to the judicial overreliance on DNA and weak investigation could never be recovered, despite the use of cutting-edge technology. Nonetheless, secondary transfer doesn’t always have to lead to false prosecution if it’s acknowledged and acted upon accordingly. For instance, the use of Bayesian networks allows scientists’ subjective propositions regarding how or from whom the genetic material had been deposited to be supported by logical reasoning and combined with empirical data. Bayesian networks recognize the relationship and conditional dependencies between different variables, along with computing a likelihood ratio. An LR value equal to 1 is interpreted as neutral. As the value increases above one, the evidence support for the prosecution hypothesis strengthens. However, if it decreases below 1, the support for the defense hypothesis becomes stronger. The insight acquired from the probability can be useful in the jury’s decision-making process, paving the way to an accurate conviction.

Now, as a veteran forensic scientist; you know that secondary transfer DNA can cause complicated situations to occur in fields such as forensics and law, leading to misinterpretation of genetic material and false action in court. However, it is preventable with a correct approach and appropriate methods. Acknowledging the consequences of cutting corners or taking the easy way out by ignoring the steps for accurate DNA profiling can harm individuals mentally and physically, sometimes causing irreversible damage. This is why it isn’t something that should be taken lightly. You reminisce about your first case, where you believed the perpetrator was the victim’s sister. If you had concluded your investigation there, the truth would have been overlooked. However, you weren’t misdirected by secondary transfer DNA, allowing justice to find its place. Resources:1: Lacerenza, D., et al. (2016) A molecular exploration of hDNA/RNA extracted from the palmar surface of the hands and fingers. Forensic Science International: Genetics. 2: Goray, M., et al. (2010) Secondary DNA transfer of biological substances under varying test conditions. Forensic Science International: Genetics. 3: Jansson, L., et al. (2022) Individual shedder status and the origin of touch DNA. Forensic Science International: Genetics. 4: Gill, P. (2017) How misuse of DNA evidence has led to miscarriages of justice. 5: Kulick, K. (2015) How an Innocent Person’s DNA Turned Up at a Murder Scene: Case Study of Lukis Anderson. American Academy of Forensic Sciences. 6: Schaapveld, T. E. M., et al. (2018) Bayesian networks for the interpretation of biological evidence. WIREs Forensic Science Volume 1, Issue 3.