The use of DNA profiling in rape investigations

DNA profiling is the use of genetic information to match crime scene evidence with suspects in criminal investigations (Easteal & Easteal, 1990). Introduced in the late 1980s, this technique has revolutionized forensics laboratories as it allows scientists to reach a certain degree of identification as well as exclude individuals from inquiries (Easteal & Easteal, 1990). With the exception of monozygotic twins, genetic comparisons are based on the uniqueness of each individual’s DNA profile. This characteristic allows DNA bands to be compared to the crime scene evidence and ascertain the identity of suspects.
In order to employ DNA evidence in investigations, the small DNA samples obtained from the suspects and the crime scene need to be amplified using PCR. Also known as Polymerase Chain Reaction, it is a technique through which DNA is forced to replicate artificially to obtain larger samples of the same molecule. This is done by heating up the strands – which are forced to separate – and using Taq polymerase to extend the newly created DNA molecules (Figure 1).

 

dna1Figure 1: Polymerase Chain Reaction (PCR) (article). Retrieved from https://www.khanacademy.org/science/biology/biotech-dna-technology/dna-sequencing-pcr-electrophoresis/a/polymerase-chain-reaction-pcr

 

After being amplified, the DNA samples are cut at specific points using restriction enzymes, which generate DNA fragments (Cornell, 2016) (Figure 2).

DNA4

Figure 2: Restriction Enzymes. Retrieved from
https://moodle.clsd.k12.pa.us/district_videos/Biology/iText/products/0-13-115540-7/ch13/ch13_s2_1_pr.html

 

Gel electrophoresis is then used to classify the DNA fragments, which are separated according to size. This allows the samples to be compared to the crime scene evidence and, eventually, convict suspects. For this to happen, there must be a 100% match of the suspect’s DNA with the DNA sample taken from the crime scene (Cornell, 2016) (Figure 3).

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Figure 3: Gel electrophoresis. Retrieved from https://commons.wikimedia.org/wiki/File:Gel_Electrophoresis.svg

 

Figure 4 shows different DNA patterns from a criminal investigation which includes three suspects.

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Figure 4: DNA fingerprinting. Retrieved from https://geneed.nlm.nih.gov/topic_subtopic.php?tid=37&sid=38

The first pattern on the left, which is surrounded by a yellow line, is the DNA sample taken from the crime scene. As a 100% band match is always required in forensics, it can be concluded that the evidence originates from suspect 2.
A common application of DNA profiling in criminal cases is the identification of rapists. The protein-based methods which were previously used were usually limiting, as both semen and vaginal fluids have enzyme and a blood group activity. Hence, mixtures of the two often occurred. On the other hand, experimentation with DNA samples simplified the purification of the male component by allowing to separate the sperm from the other cellular material (Gill, Sparkes, & Tully, 2001).
The first time that DNA profiling was employed to provide evidence for court cases was in 1989. A man, who was charged with three counts of sexual assault, changed his defence from “I wasn’t there” to an affirmation that the woman consented to the act. This change was motivated by profiling evidence which matched his blood with crime scene evidence (Easteal & Easteal, 1990). In this case, DNA profiling allowed the court to identify an individual as a certain suspect and proceed with the trial, which included considering additional evidence to reach a verdict.
However, is DNA profiling any limited? If yes, in what ways?

Although DNA profiling is – to a great extent – a scientifically reliable technique, it has some limitations. Firstly, extracting a DNA sample from crime scenes is not always straightforward. In fact, the amount taken must be adequate, as well as uncontaminated from bacteria or fungus and undegrated from sunlight, heat and humidity. Difficulties may as well arise from the DNA of the suspects. For instance, if a rapist has had a vasectomy – which is a surgical process for male sterilization (vasectomy. 2018. In Merriam-Webster.com. Retrieved from https://www.merriam-webster.com/dictionary/vasectomy) – before committing the assault, his semen would contain little or no DNA (Easteal & Easteal, 1990). Furthermore, cultural influences play a key role in rape investigations. Because of how sexual assaults might be perceived by society, many victims might not report the crime or report it too late for effective fluids to be obtained (Easteal & Easteal, 1990). Other key limitations include the aim of rape investigations itself. In most cases, whether the sexual intercourse happened is not matter of discussion; the main question often remains the victim’s consent (Easteal & Easteal, 1990), which cannot be determined by analysing or comparing DNA samples. Limitations regarding individuals having identical DNA sequences are usually not considered as such, since the probability of this happening is infinitesimally small.
To conclude, the application of DNA profiling in forensics, and in rape investigations specifically, has been extensive. According to Sumanta et al. (2017), it is no coincidence, in fact, that “DNA based identification from the biological exhibits in forensics is considered as the most important evidence for legal proof in courts” (Sumanta, Yadav, Bara, Sinha, & RS, 2017). However, one must never neglect the context of the investigation and integrate the additional legal evidence gathered in order to develop a suitable trial strategy as well as, in the case of juries, a final verdict.

 

 

Bibliography
Cornell, B. (2016). DNA Profiling. Retrieved from bioninja: http://ib.bioninja.com.au/standard-level/topic-3-genetics/35-genetic-modification-and/dna-profiling.html
Easteal, P. W., & Easteal, S. (1990). The Forensic Use of DNA Profiling. Canberra: Australian Institute of Criminology.
Gill, P., Sparkes, R., & Tully, G. (2001). DNA Profiling in Forensic Science.
Sumanta, Yadav, V. K., Bara, N., Sinha, H. K., & RS, S. (2017). DNA Profiling of a Rape Case at the State Forensic Science Laboratory Ranchy Jharkhand India. Ranchi: Austin Journal of Forensic Science and Criminology.
vasectomy. 2018. In Merriam-Webster.com. Retrieved from https://www.merriam-webster.com/dictionary/vasectomy .

 

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