This paper examines the role of DNA evidence in forensic science and criminal investigations. Beginning with the biological basis for DNA uniqueness across individuals, it traces the development of DNA fingerprinting from Alec Jeffreys's 1985 breakthrough to its widespread adoption in law enforcement. The paper discusses the range of biological substances and physical items from which DNA can be collected at crime scenes, outlines the strict protocols required for proper collection, storage, and chain-of-custody maintenance, and explains how contamination or degradation can render evidence inadmissible. Finally, it addresses the inherent limitations of forensic DNA analysis, particularly the probabilistic nature of DNA matching and the concept of the likelihood ratio.
Organisms on Earth use DNA as their genetic blueprint, and each individual organism — including humans — has a DNA sequence unique to that individual. Even within the same species, which shares a high percentage of DNA similarity, each individual still possesses a unique DNA profile. With the advent of DNA technology, the science of forensics has incorporated DNA analysis into the identification of criminal suspects with increasing regularity. How a suspect's DNA is discovered, linked to a crime scene, and used as a means of identification is therefore a central concern in forensic science.
Each individual organism possesses a unique DNA signature that can be used as a means of identification. By scanning 13 separate DNA regions within the genome, it is possible to "create a DNA profile of that individual (sometimes called a DNA fingerprint)" ("DNA Forensics"). DNA fingerprinting was first investigated by Alec Jeffreys, an English scientist who, in 1985, developed a technique to use DNA as a means of performing human identification ("History of Forensic DNA Analysis"). Since that time, the use of DNA for identification purposes has grown exponentially, especially among law enforcement, and there are currently hundreds of laboratories that perform DNA identification for purposes ranging from paternity tests to criminal investigations.
In the realm of forensics, DNA has been successfully collected and analyzed from a wide variety of biological substances, including blood and bloodstains, semen, tissues and cells, bones and organs, hairs and hair follicles, as well as urine and saliva (Catalin, p. 2). These materials can be recovered from items such as weapons, hats, bandannas, eyeglasses, dirty laundry, used cigarettes, stamps or envelopes, used condoms, bed linens, and undergarments, or from the body of a crime victim — as in the case of rape (Catalin, pp. 3–4).
It is important that these samples be collected as soon as possible, given the risk of contamination or degradation caused by prolonged exposure. In the case of blood, for example, liquid samples must be stored with an appropriate anti-coagulant to maintain the blood's liquidity. Other blood samples must be dried if still wet, then placed separately in their own sealed containers, labeled with the date and location of collection, and stored in a suitable environment — a refrigerator or freezer, depending on the type of sample. Proper handling from the moment of collection is essential to preserving the integrity of forensic evidence.
"Protocols required to keep evidence admissible"
"Contamination risks and probabilistic matching constraints"
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