What Is Touch DNA and How Is It Used in Forensics?
Touch DNA is a method that has broadened the scope of biological evidence in criminal cases. This technique operates on the premise that individuals leave behind microscopic genetic material simply by making contact with an object. It offers a way to identify individuals even when traditional biological samples like blood or saliva are not present at a scene.
What is Touch DNA?
Touch DNA, also known as trace DNA or low copy number DNA, refers to the minuscule amounts of genetic material left behind when a person handles or comes into contact with an item. This DNA primarily originates from epithelial cells, which are skin cells shed naturally from the body’s surface. Humans shed approximately 400,000 skin cells daily, and these cells contain enough genetic material to create a DNA profile.
These shed cells can adhere to various surfaces. The amount of DNA deposited can be influenced by factors such as the duration and pressure of contact, and an individual’s propensity to shed cells. Touch DNA analysis can generate a profile from as few as 5 to 30 skin cells.
Collection and Analysis
Collecting touch DNA evidence at a crime scene requires careful and precise techniques due to the minute quantities involved. Common collection methods include swabbing, where sterile cotton or nylon swabs are used to wipe surfaces, and tape lifting, which involves pressing adhesive tape onto an object to pick up shed cells. Sometimes, moistening the swab with sterile water can increase the efficiency of DNA recovery, particularly from human skin. After collection, samples are carefully packaged to prevent contamination and degradation, then transported to a forensic laboratory.
Once in the lab, the analysis process begins with DNA extraction, which isolates the genetic material from the collected cells. The extracted DNA is then quantified to determine its amount and purity, ensuring enough material for further analysis. Subsequently, the DNA undergoes amplification using Polymerase Chain Reaction (PCR), a process that creates millions of identical copies of specific DNA regions, making even tiny amounts detectable. The amplified DNA is then profiled using Short Tandem Repeat (STR) analysis, which examines highly variable regions of DNA unique to each individual. The resulting DNA profile can then be compared to known samples from suspects or entered into forensic databases like CODIS (Combined DNA Index System) to search for potential matches.
Forensic Applications
Touch DNA enables investigators to link individuals to crime scenes or objects they may have only briefly touched. For example, DNA profiles can be obtained from items such as weapons, tools, clothing, or door handles, providing leads. This technique is useful in property crimes, like burglaries or vehicle thefts, where perpetrators often leave behind only trace amounts of DNA.
The ability to recover and analyze minute quantities of genetic material has also provided new avenues for re-examining cold cases. Evidence previously deemed insufficient for DNA analysis can now be re-submitted to laboratories, potentially yielding profiles that can identify suspects or exonerate wrongly accused individuals. Touch DNA can also help reconstruct crime scene events by linking individuals to specific objects, even if no direct contact was observed.
Challenges and Limitations
Touch DNA analysis presents several challenges due to the extremely small quantities of DNA involved. This minute amount makes samples highly susceptible to contamination, where foreign DNA from investigators, laboratory personnel, or the environment can inadvertently mix with the evidence. Even a sneeze or cough can introduce contaminating DNA. Strict protocols, including specialized personal protective equipment and dedicated “clean” laboratory spaces, are necessary to mitigate this risk.
Another challenge is secondary transfer, where DNA is transferred indirectly from one person to an object via an intermediary. For example, a person’s DNA could be transferred to an object they never touched if someone else who had their DNA then handled that object. Studies have shown that secondary transfer can occur, potentially leading to false associations. It can be difficult for forensic scientists to distinguish between primary (direct) and secondary (indirect) DNA transfer, which complicates the interpretation of results.
Determining the precise timing of DNA deposition and the specific activity that led to its transfer remains challenging. The mere presence of DNA does not necessarily indicate direct involvement in a crime, as DNA can persist on surfaces for varying durations depending on environmental factors like temperature, humidity, and UV light exposure. This highlights that touch DNA evidence requires careful interpretation within the broader context of other evidence to ensure accurate conclusions.