DNA profiling is a scientific technique used to identify individuals based on unique patterns found within their genetic material. The process analyzes specific, highly variable regions of deoxyribonucleic acid (DNA) to create a distinctive numeric profile for an individual. This method, sometimes referred to as genetic fingerprinting, revolutionized forensic science by providing a powerful tool for linking biological evidence to a single person. The technique relies on the principle that, with the exception of identical twins, no two people share the exact same genetic code.
The Scientific Basis of Profiling
A DNA profile is created by focusing on specific, non-coding segments of the genome known as Short Tandem Repeats (STRs). These STRs are short sequences of DNA, typically two to five base pairs long, that are repeated multiple times at various locations, or loci, on a chromosome. Every individual inherits a specific number of repeats at each locus from their parents. This number varies widely between unrelated people, providing the basis for individualization.
Generating a profile begins by extracting a small amount of DNA from a biological sample, such as blood, saliva, or skin cells. Due to the minute nature of forensic samples, a technique called Polymerase Chain Reaction (PCR) is used to amplify the target STR regions. PCR creates millions of copies of the specific STR sequences, which is necessary to generate a readable profile.
Once the selected STR regions are amplified, the resulting DNA fragments are separated and measured using capillary electrophoresis. This process separates the fragments based on their length; fragments with more STR repeats travel slower than those with fewer repeats. A laser detects a fluorescent tag attached to the fragments, and a computer converts the data into a graph called an electropherogram. The peaks on this graph represent the number of repeats at each tested locus, creating the individual’s unique numeric DNA profile.
Primary Uses of DNA Profiling
The primary application of DNA profiling is in forensic science, where it provides a reliable means of linking individuals to crime scenes. Biological evidence collected from a scene can be profiled and compared against a suspect’s profile to establish a connection or to exclude an innocent person. It is often used in criminal investigations, including resolving cold cases years after the initial crime occurred.
Beyond the criminal justice system, the technique is widely used for establishing biological relationships, most commonly in paternity testing. Because an individual inherits one copy of each STR locus from each parent, the resulting profile is a composite of the parents’ profiles. By comparing the child’s profile to the presumed father’s, laboratories can determine the probability of a biological relationship.
DNA profiling is also used for identifying human remains, especially in mass disaster victim identification (DVI) efforts. When traditional methods fail due to fragmentation or decomposition, a profile can be generated from post-mortem samples like bone or teeth. This profile is then compared to ante-mortem samples, such as medical records or personal items, or to profiles from immediate family members to confirm identity.
National DNA Database Systems
To maximize the effectiveness of DNA profiling, national computerized systems have been developed to store and compare profiles on a large scale. These systems, like the Combined DNA Index System (CODIS) in the United States, enable law enforcement to exchange and compare profiles across different jurisdictions. Standardization is achieved by requiring all participating laboratories to analyze a specific, agreed-upon set of STR loci, ensuring that profiles are comparable regardless of where they were generated.
The digital records within these databases are typically separated into distinct indices based on the source of the DNA. The Offender Index contains profiles generated from known individuals convicted of qualifying offenses. The Forensic Index contains profiles developed from unknown individuals from biological evidence collected at crime scenes.
When a new profile is entered into the system, the software automatically searches for matches. This process can link an unknown crime scene profile to a known offender, known as a “cold hit.” Matches can also link multiple crime scenes to one another, suggesting a serial offender is involved. The database only stores the numeric profile and non-identifying information, such as the submitting agency, not personal names or criminal history.
Understanding Reliability and Limitations
DNA profiling is an accurate method, yet it is not immune to technical limitations, which primarily involve the quality of the sample and the statistical interpretation of the results. One limitation is the potential for sample contamination, which can occur at the crime scene, during evidence handling, or within the laboratory itself. Because modern techniques are sensitive enough to detect minute traces of DNA, foreign genetic material can be inadvertently introduced, leading to a mixed or misleading profile.
The challenge of interpreting results increases when dealing with degraded or mixed samples. Degraded DNA, often exposed to environmental factors like heat or moisture, can result in “allele dropout,” where one or both of the alleles at a locus fail to be detected. A mixed sample, containing DNA from two or more contributors, can be challenging to resolve, especially if the contributors provided similar amounts of DNA.
Even with a full, clear profile, a match is presented not as a certainty but as a statistical measure called the Random Match Probability (RMP). The RMP is the estimated chance that a randomly selected, unrelated person from the population would happen to have the exact same profile. This probability is calculated by multiplying the frequency of each individual allele across all tested loci, often resulting in probabilities such as one in a quadrillion.