How did the DNA of a suspect get to the crime scene? A practical study in DNA transfer during lock-picking

Increasingly in courts, the source of forensic DNA trace evidence is not challenged by the defence. Instead, the issue relates to how the suspect’s DNA got to the crime scene, suggesting it arrived via a legitimate activity. Many burglaries are committed using a new lock-picking method called ‘foil impressioning’, which involves inserting aluminium foil into the lock. When DNA is recovered from this foil, suspects will say that they only cut the foil and sell or discard it, to be used later in lock-picking by someone else. This study elucidates the probabilities of recovering DNA profiles from aluminium foil in three situations: (1) when a person only cuts the foil, (2) when they cut and then use it for lock-picking and (3) when they cut it and someone else uses it for lock-picking. Using a Bayesian network, we conclude that the probability of recovering a person’s DNA profile from foil inside a picked lock is seven times higher if they picked the lock than if they only cut the foil. This study demonstrates how likelihood ratios based on probabilistic models of DNA transfer and persistence can now help the court address questions about the mode of deposition of DNA traces.

在当今司法庭审中，此类情形愈发普遍：辩方往往不再对法医DNA微量证据的来源提出质疑，转而关注嫌疑人的DNA如何抵达犯罪现场，即该DNA可能通过合法活动附着至现场。当前诸多入室盗窃案件采用一种名为‘箔片压印开锁’（foil impressioning）的新型开锁手法，其操作流程为将铝箔插入锁具内部。若从该铝箔上检出DNA，嫌疑人通常会辩称自己仅裁剪过该铝箔，并将其售卖或丢弃，后续由他人使用该铝箔实施开锁行为。本研究明确了三种场景下从铝箔上检出DNA分型（DNA profile）的概率：（1）人员仅裁剪铝箔；（2）人员裁剪铝箔后自行用其开锁；（3）人员裁剪铝箔后由他人用其开锁。借助贝叶斯网络（Bayesian network）分析，本研究得出结论：相较于仅裁剪铝箔的情形，若人员自行使用该铝箔开锁，则从该锁具内铝箔上检出其DNA分型的概率为前者的7倍。本研究证实了基于DNA转移与留存概率模型构建的似然比（likelihood ratio），可如何助力法庭解答DNA微量证据的附着方式相关问题。