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Monday Article #46: Forensic DNA Fingerprinting in Exoneration


In 1984, 22-year-old Jennifer Thompson had her apartment broken into and was raped. During the entire assault, she made sure to remember her attacker’s facial features and physical attributes. After lodging a report to the police, they could generate a composite sketch of the attacker (Weir, 2016). The police gathered the potential suspects and presented them in a six-man line-up to Thompson. With 100% certainty, Thompson pointed out one of the men to be her attacker and quoted ‘Looking at a series of police photos, I identified my attacker. I knew this was the man. I was completely confident. I was sure” (Locard’s Lab, 2015).

The attacker was identified to be Ronald Cotton who had a criminal record at 16 years old for attempted rape. Due to such circumstantial evidence, Cotton was convicted and sentenced to life imprisonment. While in prison he meets another inmate named Bobby Poole who was convicted of rape. Coincidentally, both Cotton and Poole shared several resembling features which often resulted in the prison staff misidentifying them for one another (Locard’s Lab, 2015). Given this discovery, Cotton remained optimistic, however during his second trial, the jury did not believe that Poole could have been the attacker and had to carry out his life sentence (O’Neill, 2001).

Figure 1: Side-by-side comparison of Ronald Cotton (left) and Bobby Poole (right)

Source: (Ryan, 2015)

Thankfully in 1994, Cotton was made aware of DNA testing which was a laboratory procedure that was unavailable during his time of conviction. By 1995, the semen and other bodily fluids that were collected at the scene of the crime in 1984 were DNA tested. DNA testing revealed that the semen did not match Cotton and was instead a match for Poole. Therefore after nearly 11 years in prison, Cotton was exonerated and cleared of all charges (O’Neill, 2001). However, the negative implications of what happened may forever affect Cotton.


As proven in the previously stated case study, eyewitness testimony is often fragile and prone to mistakes. However, eyewitness testimony is extremely viable and useful in forensic and legal aspects. Therefore the use of DNA testing should also be applied in order to obtain the most cohesive result to produce a justified verdict.

In 1994, the NDIS (National DNA Index System) was created, this is a system consisting of all registered DNA profiles of criminals or arrested individuals. Such deposited DNA profiles are known as forensic DNA. thus enhancing investigative advances and determining crime trends of repeated offenders. Forensic science involving DNA evidence aids in: eliminating suspects, convicting criminals, exonerating inmates, and reducing false convictions (Wickenheiser, 2021).


The first step in DNA fingerprinting is to obtain a DNA-rich biological sample from the scene of the crime; such include blood, hair, semen, saliva, nail clippings, etc (yourgenome, 2021). The sample is then introduced to specific restriction enzymes, which will cleave the DNA at specific recognition sites. These DNA fragments are of varying lengths and are generated via Restriction Fragment Length Polymorphism (RFLP) (Nayak, 2023). To provide context, DNA (Deoxyribonucleic acid) are fundamental building blocks of all body cells. Within the DNA sequences, there are unique identifiers, special to each individual. Such identifiers are known as Short-Tandem Repeats (STRs) which are a series of repetitive sets of base pairs that may span up to 100 nucleotides long. STRs make up around 3% of the entire human genome and are spersed around throughout the entire length of the genome (Fan and Chu, 2007).

After which the DNA in the fragments is to be isolated and extracted via chemical treatment and centrifugation (Nayak, 2023). The isolated DNA then needs to be amplified, this is achieved via the Polymerase Chain Reaction (PCR) which is the ideal method due to its high compatibility with STR markers (Stanley, 2020). The amplified DNA fragments are then loaded into the wells of the agarose gel which have been placed within the electrophoresis tank. The agarose gel is porous and therefore permeates movement which is stimulated by the electrical current. The negative electrode is at the end where the wells containing the samples are, and vice versa the positive electrode is at the end. The applied electrical current causes the DNA fragments to move across the gel, with the smaller fragments traveling faster (yourgenome, 2021).

Figure 2: Illustration / Diagram of the DNA profiling procedure

Source: (Nayak, 2023)

The separated DNA fragments/banding patterns on the gel then needs to be transferred onto either a nylon or nitrocellulose membrane. This process is known as southern blotting, whereby the nylon membrane is placed on top of the gel, absorbing the banding pattern. The nylon membrane then undergoes hybridization beginning with being submerged in a bath containing radiolabeled probes, thus tagging the DNA fragments with a radioactive isotope (Nayak, 2023). Afterward, the hybridized nylon membrane is exposed to X-ray film to reveal the unique banding patterns in the form of light and dark bands (yourgenome, 2021).

Referencing back to the case study, the generated DNA profile of a crime scene sample can be compared to the suspected individual. If the individual is guilty, both DNA profiles should display a 100% match. In the case of Ronald Cotton, his DNA sample did not match that of the sperm sample obtained at the scene of the crime, and thus was exonerated. However, the lack of such scientific advances during his time of conviction resulted in a trial backed solely by a confident eyewitness and the faultiness of human memory. Ronald Cotton was not the only individual that suffered from false accusations, numerous others fell victim due to DNA testing being unavailable at the time. As of 2020 in the United States, DNA testing has managed to exonerate 375 individuals, with 21 of them who were placed on death row. Amongst these people, they served an average of 14 years in prison prior to being exonerated. The long-term implications of such unjust imprisonment can’t be remedied immediately, however, the use of DNA testing in exonerating the innocent is a step in the right direction (Innocence Project, 2020).


  1. Fan, H., & Chu, J. Y. (2007). A brief review of short tandem repeat mutation. Genomics, proteomics & bioinformatics, 5(1), 7–14. Retrieved from:

  2. Innocence Project. (2020). Exonerate the Innocent. Retrieved from:

  3. Locard’s Lab. (2015). Forensic Failures: Eyewitness Testimony & The Ronald Cotton Trial. Retrieved from:

  4. Nayak, J. (2023). DNA Fingerprinting: Steps, Process, & Importance. Retrieved from:

  5. O’Neill, H. (2001). The Perfect Witness. Death Penalty Information Center. Retrieved from:

  6. Ryan, B. (2015). Eyewitness Testimony is Unreliable…Or Is It?. Retrieved from:

  7. Stanley, U.N., Khadija, A.M., Bukola, A. T., Precious, I.O., & Davidson, E.A. (2020). Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation. The Malaysian journal of medical sciences: MJMS, 27(4), 22–35. Retrieved from:

  8. Wickenheiser R. A. (2021). The value of forensic DNA leads in preventing crime and eliminating the innocent. Forensic science international. Synergy, 3, 100201. Retrieved from:

  9. Weir, K. (2016). Mistaken identity - Is eyewitness identification more reliable than we think?. American Psychology Association. Retrieved from:

  10. yourgenome. (2021). What is a DNA fingerprint?. Retrieved from:


This article was prepared by Jocelyn Chia Li Yen


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