Touch DNA is a forensic term for the minute genetic material transferred when a person’s skin cells are shed onto an object through simple contact. This evidence has gained significance in modern forensic science because technological advancements allow for the identification of individuals from minimal biological evidence. The ability to recover a DNA profile from an item that was only briefly handled, such as a weapon, a garment, or a door handle, has expanded the scope of criminal investigations. Touch DNA is a powerful tool for linking a person to an object or a location, even in the absence of traditional biological fluids like blood or semen.
How Trace DNA is Left Behind
The deposition of trace DNA relies on the constant shedding of epithelial skin cells from the body, along with cell-free DNA contained within biological secretions like sweat and sebum. A significant factor influencing the amount of DNA left behind is an individual’s “shedder status,” which categorizes people as high, intermediate, or low shedders. The exact biological mechanisms behind this difference remain largely unknown.
The amount of deposited DNA is also affected by the nature of the contact, the surface material, and the condition of the skin. Studies suggest that the DNA on a person’s hands often accumulates from other parts of the body, such as the face, via subconscious touching. This means the genetic material recovered from a touched object may not strictly be skin cells from the hand itself, complicating the interpretation of the source.
High-Sensitivity DNA Analysis
Analyzing Touch DNA requires specialized laboratory techniques because the recovered samples are often degraded or present in extremely low quantities, frequently less than 100 picograms. These samples are classified as Low Copy Number (LCN) DNA, which is roughly equivalent to the genetic material from only about 15 human cells. Standard DNA profiling methods, optimized for larger, more robust samples, typically fail to generate a usable profile from these minute amounts.
To analyze LCN samples, forensic laboratories use modified procedures that significantly increase the sensitivity of the Polymerase Chain Reaction (PCR) amplification process. This involves increasing the number of thermal cycles used to copy the DNA segments, which helps boost the allelic signal intensity. The enhanced sensitivity, however, can also introduce stochastic effects, leading to challenges in obtaining a full, balanced DNA profile, often resulting in a partial profile that is more difficult to interpret.
Understanding Secondary and Tertiary Transfer
The fundamental challenge to the reliability of Touch DNA is indirect transfer, meaning the presence of DNA does not necessarily prove direct contact with an object. Secondary transfer occurs when DNA moves from person A to person B, and then from person B to an object. For example, if two people shake hands and one then handles a weapon, person A’s DNA may be detected on the weapon without A ever having touched it.
Tertiary transfer is a further complication, where DNA moves from person A to object 1, then to an intermediate vector like a glove or another person, and finally to object 2. The ease with which DNA can be passively transferred means a DNA profile matching a suspect could have arrived at a crime scene through an innocent vector, such as a first responder’s glove or a commonly shared tool. In some laboratory studies, the amount of DNA transferred secondarily has occasionally surpassed the quantity deposited by primary, direct contact. The scientific consensus acknowledges that while the technology can reliably identify whose DNA is present, it cannot definitively determine how or when the DNA was deposited.
Admissibility in Criminal Cases
The admissibility and weight given to Touch DNA evidence depend heavily on the legal jurisdiction’s standards for scientific evidence. Courts must grapple with the scientific implications of secondary and tertiary transfer when assessing the probative value of a DNA match. The legal question has shifted from simply confirming the identity of the person whose DNA was found to determining the activity that led to its deposition.
Expert witnesses play a significant role in interpreting the significance of a match, particularly when the profile is partial or the amount of DNA is extremely low. They must explain to the court that while a match is scientifically valid, the possibility of innocent transfer must be considered as a reasonable alternative explanation. Consequently, while Touch DNA can be powerful corroborating evidence, its presence alone is often insufficient to establish guilt without other contextual evidence linking the individual to the criminal activity.