Deoxyribonucleic acid (DNA) plays a vital role in forensic science through exonerating the innocent and convicting the guilty. The genetic material in DNA allows the identification of the perpetrator by the processing and the analysis of biological evidence transferred in the crime scene.
DNA Evidence at Crime Scene. Image Credit: felipe caparros/Shutterstock.com
These biological materials take a variety of forms from buccal swabs, saliva, semen, blood, vaginal swabs to touch DNA. The materials are sometimes deposited in trace amounts on a surface without knowledge from the perpetrator.
Through the processing of DNA from biological fluid samples, alongside DNA statistical interpretation, the identification of an individual can be done. This is done when swabbing known profiles from the scene, and through DNA databases, such as Combined DNA Index System or CODIS. CODIS contains reference profiles from past offenders, can be used to search for the questioned profile when a known profile is not available.
Human Identification before DNA
DNA profiling or genetic fingerprinting for forensic and human identification purposes had only begun in the early 1980s. This method of identification was discovered when the British geneticist, Sir Alec John Jeffreys observed the similarities and distinguishing characteristics of DNA within a family in his lab in Leicester. This method was then first used to resolve an immigration case in determining the genetic relationship of a British boy to his family members with Ghanaian origins.
With the efficiency and the reliability of this scientific method, DNA fingerprinting was used for police work in identifying the murder cases of Lynda Mann and Dawn Ashworth, the first criminal case of many that are solved through the power of DNA. Nevertheless, other genetic markers, blood group testing, and protein profiling were the best methods at the time to exclude individuals as contributors from biological specimens at a crime scene.
The four blood types were identified by the Austrian researcher, Karl Landsteiner in 1900 with 42% of the population observed as type A, 12% as type B, 3% as type AB and 43% as type O. The ABO blood types were the first genetic evidence presented in court for paternity disputes and forensic purposes in Europe and the United States.
With antigen polymorphisms existing on red blood cells, with distinguished antigens in protein, carbohydrate, glycolipid, or glycoprotein that are present between individuals, alongside the inheritance of these antigens from the parents, the blood group testing was generally used for paternity testing. The blood group antigenic alleles were interpreted by developing antibody-based serological tests.
Nonetheless, the laboratory tests involving serology are still implemented in many forensic laboratories to detect the alleles of ABO as well as other blood groups, MN and Rh through examining antigen-antibody interactions by immunological assays. Furthermore, the characterization assays in blood, saliva, sperm, and semen can also be done through serology testing.
Forensic protein profiling was another method developed in the middle to late 1980s to differentiate between questioned and known samples in forensic laboratories. The discrimination power to differentiate two individuals rely on the variety of amino acid sequences in the proteins that differ between individuals. The alleles are distinguished by separating the proteins through methods such as agarose gel, polyacrylamide gel, and starch gel electrophoresis.
However, even with more advanced methods developed such as the isoelectric focusing polyacrylamide gel electrophoresis and silver staining that can detect a low amount of proteins in biological samples, proteins are less stable and not as varied as using DNA for forensic investigations.
DNA as a powerful human identification tool
In today’s age, hundreds of forensic laboratories in both the public and private sectors, alongside paternity as well as paternity testing laboratories utilize hundreds of thousands of DNA tests in North America alone. Computer databases with DNA profiles compiled from convicted offenders, biological samples swabbed from crime scenes, and individuals arrested for a crime, have tremendously aided law enforcement to solve crimes and assign sanctions to offenders for the specific crimes committed.
Furthermore, forensic DNA testing has done more than just punishing the guilty for the crimes they have committed. The piece of evidence has also exonerated the innocents from crimes they did not commit. Wrongfully convicted prisoners from the result of other types of evidence or faulty witnesses were found guilty before the implementation of DNA typing methods.
Nevertheless, through the power of postconviction DNA testing from specimens that were preserved for many years, more than 200 of these individuals that included death row inmates were exonerated. Through the successes of utilizing DNA as evidence in court to prosecute the guilty and exonerate the innocent for decades, DNA is repeatedly sought by judges, attorneys, and detectives as primary evidence from a crime scene.
The meaning behind a ‘DNA match’
When DNA profiles from questioned and known samples are available, one of the three possible outcomes is stated in a report: inclusion, exclusion, and inconclusive. Inclusion or a match indicates that the same genotypes are observed on the peaks between the two short tandem repeat (STR) profiles and the technical differences between the profiles can be explained.
Exclusion or a nonmatch explains that the genotypes within the profiles differ and this represents that the samples originate from different sources. While inconclusive indicates that there is insufficient information to lead to a conclusion and if two DNA analysts are in disagreement during the technical review process.
In addition, a ‘match’ can indicate one of three possibilities; the suspect deposited the sample, the suspect did not deposit the sample but there is a false positive caused by the laboratory processing, or the suspect did not deposit the sample but happened to have the sample. Nonetheless, regardless of how many loci match, when one STR locus does not match when two genotypes of the samples are compared, the questioned and the known samples are reported as a nonmatch.
- Butler, J. (2010). Fundamentals of forensic DNA typing. Amsterdam: Academic Press/Elsevier.
- Ballantyne, J. (2000). Serology: Overview. In J.A. Siegel, et al. (Eds.), Encyclopedia of forensic sciences. 3rd ed. pp. 1322 – 1331.
- Scientific Working Group on DNA Analysis Methods. (2000). Short tandem repeat (STR) interpretation guidelines. Forensic Science Communication, [online] 2(3). Available at: http://www.fbi.gov/hq/lab/fsc/backissu/july2000/strig.htm (Accessed 12 May 2021)