Written by Katie Traversino and Edited by Olivia Cooper
A complete copy of our genome, our unique blueprints set composed of chemical compounds denoted as G’s, C’s, A’s, and T’s, is present in each of our cells. Epithelial cells like skin and hair are continuously regenerating and falling off of our bodies. These biological samples can easily be found at crime scenes, and the DNA they contain can be used to identify the individual(s) involved [1]. The regions of interest for criminal investigators that can identify individuals using DNA are the variable number of sequence repeats that are unique to each individual, called short tandem repeats (STRs) [2]. Forensic investigators can look specifically at these STR regions of the genome and use this information to connect a strand of hair, for example, to a human being.
DNA profiling is the identification of a person based on unique STR markers in their genome. STR regions are inherited from both parents, therefore each person has two copies [3]. Since 2017, there are 20 known locations, or loci, of STRs that scientists target for identification [3]. These unique DNA identifiers vary per person due to mutations, such as copying or deleting portions during DNA replication. Variations can also result from strand-slippage, which is when a portion of the DNA strand folds away and is skipped by replicating machinery, as well as unequal crossing over during meiosis, when chromosomes exchange their genetic material resulting in unique sequences [2].
DNA is isolated from the human samples taken at a crime scene by bursting the cells and spinning the samples to separate DNA from other materials. Then the characteristic STR regions of the isolated DNA are amplified and quantified for analysis that indicate the number of sequence repeats [3]. The goal is to quantify the number of repeats and compare this number with a sample of a known identity. If a crime scene sample consists of the same number and structure STR as the known sample, then the person represented by the sample is a valid suspect. Nonetheless, issues with DNA profiling can arise if the isolated DNA is degraded, available in small quantities, or unrelated to the crime [4]. Therefore, thorough analysis of the STR sample, along with other crime scene evidence, must be completed before indictment of an individual.
In the United States, the DNA repeats are catalogued with a database called The National DNA Index System (NDIS), or the Combined DNA Index System (CODIS) [5]. Once a STR is profiled, it is submitted to the system for comparison in the future, in case that individual commits another crime. While there is incredible potential for crime-solving, there are massive concerns about the privacy of an individual’s data and whether the government should have access. Although STR data does not immediately indict an individual for a crime, it provides strong evidence towards narrowing down suspects. Forensics science is constantly improving in accuracy and speed, and the efficacy has been shown in the number of complicated cases that DNA profiling has solved over the past thirty years, bringing peace to many families.
References:
- Diegoli, T. M. (2015). Forensic typing of short tandem repeat markers on the X and Y chromosomes. Forensic science international. Genetics. 18:140–151.
- Nwawuba Stanley, U., Mohammed Khadija, A., Bukola, A. T., Omusi Precious, I., Ayevbuomwan Davidson, E. (2020). Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation. The Malaysian journal of medical sciences : MJMS, 27, 22–35.
- Arnaud, Celia. “Thirty years of DNA forensics: How DNA has revolutionized criminal investigations.” Chemical and Engineering News. ACS. 2017. Accessed 17 Jan 2021. https://cen.acs.org/analytical-chemistry/Thirty-years-DNA-forensics-DNA/95/i37.
- Norrgard, Karen. “Forensics, DNA fingerprinting, and CODIS.” Nature Education. 2008. Accessed 17 Jan 2021. https://www.nature.com/scitable/topicpage/forensics-dna-fingerprinting-and-codis-736/.
- Wickenheiser, R. A. (2019). Forensic genealogy, bioethics and the Golden State Killer case. Forensic science international. Synergy, 1:114–125.