Forensic is a multidisciplinary science, that helps in solving crimes. Nowadays the crime rate in the world and in India, in particular, is increasing. So the need for fast and effective methods of criminal detection is also increasing. In such cases, DNA analysis has been playing a key role to link the suspect to the victim for some time.

But in some cases, there may not found biological evidence, physical evidence, or any other visible evidence at the crime scene. In such cases, the traces of DNA transferred from the suspect during physical contact with the evidence which is found at the crime scene, help in the investigation. The transferred DNA from the suspect due to touch is called touch DNA which is also called trace DNA.

This technique has dramatically increased the number of items that can be used for DNA detection. In the 1980s, in order to perform DNA analysis, the investigators needed blood or $emen stain about the size of a quarter. However, in modern days, it only requires 7 or 8 cells from the outermost layer of our skin.

Touch DNA

Humans shed many skin cells every day and these cells may be transferred to the surfaces of our skin contacts. When a crime is committed, the assailant may leave a sufficient amount of skin cells on items found at the crime scene. If this evidence is collected and analyzed, the touch DNA may help in linking the perpetrator to the crime scene.

Touch DNA can possibly be found on victims, weapons, clothing, and vehicles. With the advent of more sensitive DNA typing kits such as MiniFiler, touch DNA profiles can now sometimes be obtained from bullet casings, documents, and latent fingerprints. The current paper studies the significance of touch DNA in solving crimes.

Methodology of Touch DNA Technology

Sampling Methods

In the research done by Angela L. Williamson, a genetic profile can easily be generated from Touch DNA collected from surfaces like glass, fabric, and plastics. However, surfaces such as metal have been proven to be difficult to recover sufficient DNA to produce a profile because metals have a range of electron affinities and ionization energies that would affect their interaction with negatively charged DNA.

The double swab technique (wet and dry) is basically used for hard, non-porous items such as glass, metals, or plastics to collect the sample. In the double swabbing technique, a sterile-cotton-tipped swab is taken from the sealed package, and placed a few drops of clean or sterile water on the cotton tip and avoid saturating the swab.

Then swab the suspected area of DNA by rotating the swab so the entire surface of the swab is used. And then re-swab that area with a dry sterile swab (use each swab only once) and placed the two swabs together in a breathable swab box and labeled the box with a specific area that was swabbed.

Another sampling technique that is frequently used in a large number of forensic laboratories is “cutting out” the area of suspected DNA. This cutting method is generally used for soft items such as clothing. In this method fabric from the areas of interest is cut to collect the DNA.

However, the double swabbing method and cutting method have the limitation of placing unnecessary substrate (the cotton swab itself or the fabric cuttings) into the processing tube.

Touch DNA extraction

In addition to the commonly used swabbing and cutting methods, several laboratories also used the scraping and tape lifting methods. In this method, the surface of porous items are either scraped with a sterile scalpel blade or sampled with a small piece of scotch tape to collect possible skin cells.

Usually, the area of damage or the area where the perpetrator is believed to have the most contact is used for this method. However, the extraction of DNA with this method is quite challenging because of the size of the tape.

DNA Isolation and Detection

After the sampling method, the isolation and extraction of DNA is carried out by the forensic scientist using chelex extraction, organic extraction, and silica-based extraction. And then polymerase chain reaction (PCR) is used to make lots of copies of the genes.

Then, the fluorescent compounds attach themselves to 13 specific locations on the gene which gives a highly specific genetic portrait of the person. These 13 locations were carefully chosen because they are highly variable between people and do not give away any specific information such as race, gender, or genetic disease and the whole process takes a few days.

Touch DNA and its Evidentiary Value

According to Angela L. Williamson’s study, continuous development of DNA extraction and testing technology, the DNA left at a crime scene plays an important role to determine the suspect in a criminal investigation. However, criminals are more aware these days. So they try to leave less amount of evidence at the crime scenes during and after the crime.

Therefore, in the process of evidence collection at the scene, collection and extraction of touch DNA are crucial. Also when examining the touch DNA, it is important that the investigator considers the potential evidentiary value of the DNA and must allow the relationship between the victim and the suspect (if one exists) and any possibility of “innocent transfer” of DNA that may have occurred before the alleged crime.

For example, if the suspect is a family member or lived with or had recent contact with the victim then the touch DNA of the suspect can easily be found because touch DNA can easily be transferred throughout the household by day-to-day interactions. To collect the touch DNA there is no need to take permission from the court, unlike the biological DNA.

In case of sexual assault by a stranger, finding the DNA of the suspect anywhere on the victim’s clothing may be of evidentiary value. In this situation, if the victim is living it is important to gather as much information from the victim as possible or to attempt to recreate the events in the case of the deceased victim.

For example, if the victim’s pants were pulled down by the perpetrator, then the investigator and the forensic scientist should consider sampling areas for touch DNA where one would imagine that the suspect would have grabbed during the assault. Collection and preservation of the clothing of victims may allow the possibility of obtaining touch DNA at a late date even if it is not initially indicated to be present at the crime scene.

The touch DNA has great potential value because even if the perpetrators try to remove their presence by not leaving any evidence at the crime scene. It is proven by Locard’s exchange principle that states “Every contact leaves a trace”, they transfer some of their DNA due to touch.

Case Studies

There are several cases that have been solved using touch DNA technology. Some of the sensational cases which were solved using this technique are as follows:

1. JonBenet Ramsey Case

Jon Benet murder case

JonBenet Ramsey was an American child beauty pageant winner, who was killed at the age of six in her house in Boulder, Colorado. Investigators found the touch DNA on the clothing of JonBenet Ramsey which she wore at the time of the murder and a full DNA profile was developed. The DNA profile was obtained from an unidentified male, presumably the murderer. Although this case still remains unsolved but touch DNA plays an important role and gives such crucial information.

2. Bodhgaya Blast Case

Bodhgaya blast case

The national investigative agency was able to crack the Bodhgaya case using the touch DNA technique. In this case, the bomber was disguised as a monk and left his clothes behind. The touch DNA recovered from the clothes could take the investigators to the perpetrator.


  • Although touch DNA has great evidentiary value in solving crime, it has some limitations. In this technique, the probable suspect should be available to match their DNA profile with the questioned touch DNA profile so that the actual culprit can be linked to the victim.
  • Touch DNA mixture interpretation is difficult.
  • Touch DNA sampling methods are very sensitive. Hence, there is a greater chance of contamination and could falsely link someone to the crime. There are several cases where touch DNA imprisons the innocent.

If these limitations are kept in mind Touch DNA can help in solving many crimes.

Contributor (forensic yard)

‘By Ayantika Biswas’

Ayantika Biswas is a student, pursuing MSc in forensic science from the College Of Traffic Management, Faridabad. She graduated from Galgotias University with a bachelor’s degree in forensic science in 2019. She did a summer internship at Codon Biotech, Delhi in 2018

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