We have heard a lot about shedding dead skin cells to the immediate environment of our body. Often not visible to our naked eyes, it does take place and is quite helpful in identifying an individual.

Forensic science has always tried to focus on trace evidence and exploit any phenomenon that can be used as evidence. Touch DNA is quite a novel term and is one of the trace evidence. Let us know more about it and dive deep into this topic.

History of Touch DNA

“Touch DNA” is DNA obtained from biological material transferred from a donor to an object or a person during physical contact. Touch DNA, also known as Trace DNA, is a forensic method for analyzing DNA left at a crime scene. It is called “touch DNA” because it only requires a minimal amount of samples, such as the skin cells left on an object after it has been touched or casually handled or from footprints.

The Locard Exchange Principle explains that, with contact between two items, there will be an exchange of materials, a concept central to the science of fingerprints and forensic sciences. The fingers act as transmission vectors, transferring sweat and oil as ridge detail which is the conventional information-bearing component of the exchange.

However, in the last two decades, forensic scientists have recognized the added capability of extracting DNA from fingerprints, i.e., sweat and oil exchange contains a second information bearing component in DNA containing cells that support genetic profiling: this is the science of touch DNA.

Touch samples contain DNA, not attributed to a particular body fluid, deposited when an object is handled or touched. The origins of the human DNA in a touch sample have not been definitively elucidated. Still, many lines of evidence indicate that they are likely to include shed corneocytes, endogenous or transferred nucleated epithelial cells, fragmented cells and nuclei, and cell-free DNA.

There is broad inter-and intra-individual variation in the quality and quantity of DNA in a touch sample. It can vary based on contrasting factors, including donor activity, gender, age, substrate, temperature, and humidity. Ergo, it isn’t easy to define an accurate fingerprint as the positive control for collecting and analyzing touch samples.

An Australian scientist first discovered the concept of obtaining and identifying information from tiny bits of touch-trace DNA and published it in 1997 under the title “DNA fingerprints from fingerprints.” The scientist discovered that little bits of DNA would transfer through touch and fingerprint markings, allowing for collecting and analyzing DNA from fingerprints left behind by culprits.

 Practical utility in the criminal justice world requires the emergence of more sensitive technologies to detect tiny bits of evidence. Such equipment began appearing in forensic laboratories over the last 20 years. But law enforcement did not limit their search for touch-transfer DNA to just fingerprints; instead, they expanded the search for touch-transfer DNA to all objects and surfaces, irrespective of the ability to find other identifying evidence connected to that DNA, such as a fingerprint.

This led to the prosecution of individuals based on DNA from low-template and low-quality samples not related to other identifying data. Moreover, prosecutors failed to distinguish the unique nature of touch-transfer DNA and the likelihood of random and innocent touch-transfer origins. Although, the Australian scientist who presented the concept of touch-transfer DNA also noted that some people’s DNA curiously appeared on things they had never touched.

The If’s and But’s

It has been observed that the perpetrator of a crime leaves traces of their DNA at the crime scene, and if found even in small amounts, the evidence can be used to identify the perpetrator. Law enforcement agencies consistently use finding a suspect’s DNA at a crime scene, on a victim, or a piece of evidence to connect a suspect with the crime scene, with the victim, or in contact with the relevant piece of evidence.

Prosecutors have been representing DNA evidence as superior to all other types of identification evidence. However, research conducted at the University of Indianapolis shows that the detection of DNA does not indicate presence or contact. In fact, it may not narrow the scope of the investigation at all.

This is because humans shed DNA continuously, and shed DNA transfers freely between people and objects. DNA can be transferred through a handshake or touching an inanimate object, like a doorknob. Every time you shake someone’s hand, you might receive some of your acquaintance’s DNA and others whose DNA had come into contact with your acquaintance’s hand.

A person identified by a match of DNA discovered at a crime scene may have never come into contact with the object or the person on which his DNA was found. Yet, modern-day high-tech Crime scene investigations do not account for the possibility that the evidence discovered at a crime scene was deposited via touch. Scientists can’t determine whether the tiny bits of DNA came into contact with evidence from a direct source or a secondary source. 

The technique has been criticized for high rates of false positives due to contamination. For example, fingerprint brushes used by crime scene investigators can transfer trace amounts of skin cells from one surface to another, leading to inaccurate results. Because of the risk of false positives, it is more often used by the defense to help exclude a suspect rather than the prosecution. There are currently no widespread standards that are specifically aimed at preventing cross-contamination with touch-transfer DNA in the laboratories where various evidentiary objects are examined.

The controversy arose partly out of confusion between DNA profiling as an investigative tool and as evidence in court. Its power as an investigative tool continues to expand. Still, its reliability as evidence will always vary depending on the exact nature of the DNA profile obtained and the state of the sample from which it was derived.

The very fact that it is possible to get DNA profiles from degraded samples means that there are borderline cases in which the accuracy of DNA profiles is doubtful. Thus, when prosecutors present to a jury touch DNA evidence with the same potency as large-sample DNA evidence, jurors, under the influence of pre-set expectations for scientific evidence to prove guilt and the common notion that DNA evidence is inherently trustworthy, feel compelled to convict. The result is that touch-transfer DNA can readily lead to a conviction of the innocent.

Touch DNA” is almost a pseudoscience. It takes samples containing a mixture of DNA from surfaces touched by an unknown number of people. It guesses the probability that a criminal suspect’s DNA is among the haystack of unknowns the lab is examining.

Although, it cannot be overlooked that advances in “touch DNA” technology make it possible for police to reopen some cold cases. So we never know what the future holds and how far the technology can go to exploit the concept of Touch DNA and yield more reliable and admissible results.


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