DNA evidence serves as a powerful tool for identification in legal settings. Its ability to uniquely link an individual to a crime scene has transformed forensic science since its introduction. The presence of DNA evidence often provides a high degree of certainty for investigators and courts. However, like any scientific method, its reliability in the courtroom is entirely dependent on the integrity of the sample and the rigor of the testing process.
The Science Behind the DNA Match
Forensic DNA analysis does not involve sequencing a person’s entire genetic code. Instead, analysts focus on specific, highly variable regions of the genome called Short Tandem Repeats (STRs). These are short sequences of DNA repeated multiple times at specific locations (loci) on a chromosome. The number of repeats at these loci varies greatly from person to person, making them ideal markers for individual identification.
The process begins by using the Polymerase Chain Reaction (PCR) to amplify the DNA at these chosen STR loci, creating a sufficient sample for testing. The resulting fragments are then separated by size using capillary electrophoresis. This separation generates a graphical output called an electropherogram, which displays the DNA profile as a series of colored peaks.
A “match” is established by comparing the number of repeats at all tested loci between the crime scene sample and a suspect’s known profile. The final result is presented with a statistical measure known as the Random Match Probability (RMP). The RMP is the probability that a randomly selected person from the population would have the exact same DNA profile, a number which is typically extremely small, often less than one in a trillion.
Securing the Evidence Chain of Custody
Before any laboratory analysis begins, the integrity of the evidence is protected through the Chain of Custody. This is a chronological, documented history that meticulously records the seizure, control, transfer, analysis, and disposition of the evidence. Maintaining an unbroken chain is paramount, as any gap in documentation can render the DNA evidence inadmissible in court by suggesting tampering or accidental substitution.
Proper collection and storage protocols are implemented immediately to preserve the biological material and prevent degradation. Wet biological evidence, such as stained clothing, must be air-dried completely before packaging to inhibit the growth of mold and bacteria. Biological samples are typically packaged in paper bags or cardboard boxes, as plastic containers trap moisture and accelerate DNA decomposition. Liquid samples, such as whole blood, are usually stored under refrigeration before submission to the crime laboratory.
Factors Affecting Sample Reliability
Despite the precision of laboratory techniques, the reliability of DNA evidence can be challenged by several factors affecting the sample itself.
Degradation
Degradation occurs when environmental conditions, such as high heat, humidity, or exposure to ultraviolet (UV) light, break the DNA molecule down into smaller, fragmented pieces. This fragmentation can lead to incomplete DNA profiles because the PCR process may fail to successfully amplify some of the larger STR regions, a phenomenon known as allele drop-out.
Contamination
Contamination involves the unintentional transfer of foreign DNA into a sample, which can happen at the crime scene, during collection, or in the laboratory. Modern, highly sensitive testing methods are particularly susceptible, as even minute amounts of transferred DNA—such as skin cells from a collector—can be amplified. This often leads to a mixed profile that complicates interpretation. The analysis of samples containing DNA from multiple individuals, known as mixtures, presents challenges in distinguishing the genetic material of the person of interest from others.
Low Copy Number (LCN) DNA
LCN DNA refers to samples containing a very small amount of genetic material, sometimes equivalent to only a few cells. While highly sensitive techniques can amplify these trace amounts, they increase the risk of errors, including the amplification of contaminant DNA or the misinterpretation of results due to stochastic effects. The potential for false results and the controversy surrounding the interpretation of these minute samples have led to challenges regarding their admissibility and weight in the courtroom.
DNA Evidence Impact on Legal Outcomes
DNA evidence has had a significant impact on the criminal justice system. The Combined DNA Index System (CODIS), a national database maintained by the FBI, allows forensic laboratories to store and compare DNA profiles from crime scenes, convicted offenders, and arrestees. This searchable network facilitates the identification of suspects and is instrumental in linking unsolved crimes across different jurisdictions, often leading to investigative leads in cold cases.
The advent of post-conviction DNA testing has become a powerful mechanism for correcting miscarriages of justice. The re-analysis of biological evidence from old cases using modern techniques has led to the exoneration of hundreds of wrongfully convicted individuals. This application of DNA technology provides irrefutable evidence of innocence in certain cases, highlighting the limitations of other traditional forms of evidence.