DNA profiling, often referred to as DNA fingerprinting, is a laboratory technique that identifies individuals based on unique patterns in their genetic material. While the vast majority of human DNA is identical, the method focuses on small, variable segments that distinguish one person from another. British geneticist Sir Alec Jeffreys pioneered the technique in 1984 after noticing highly variable DNA sequences suitable for identification. This technological approach has profoundly impacted forensic science and the justice system since its development.
The Biological Basis of Identity
The principle behind DNA profiling relies on polymorphisms, which are differences in DNA sequences among individuals. These variations occur in specific regions of the genome, typically located in non-coding DNA that does not contain instructions for making proteins. The specific genetic markers used for modern profiling are called Short Tandem Repeats (STRs). STRs are short sequences of two to seven base pairs repeated multiple times at specific locations, or loci, on a chromosome. Since every person inherits one copy of a chromosome from each parent, they possess two alleles for each STR locus. The varying number of repeats creates a highly unique combination of repeat lengths, distinguishing individuals. Identical twins are the only exception, as they share the exact same DNA profile.
Core Laboratory Techniques
The process of generating a DNA profile begins with the collection and extraction of DNA from a biological sample, such as blood, saliva, hair, or skin cells. Once extracted, the next step is Polymerase Chain Reaction (PCR), a technique used to amplify specific regions of the DNA. PCR makes millions of copies of the target STR regions, which is critical when dealing with minute or degraded samples found at a crime scene.
The amplification targets specific STR loci using specialized primers labeled with fluorescent dyes. The PCR process cycles through heating and cooling stages, which denatures the double-stranded DNA, allows primers to anneal, and extends the new DNA strand using a heat-stable polymerase enzyme. A standard set of STR loci, such as the 20 core loci used in the Combined DNA Index System (CODIS), are simultaneously amplified in this multiplex reaction.
After amplification, the resulting DNA fragments are separated and detected using Capillary Electrophoresis (CE). CE separates the fragments based on their size, with smaller fragments moving faster through a narrow, polymer-filled capillary tube under an electric field. The fluorescent dyes attached to the fragments are detected by a laser, translating the data into an electropherogram.
The electropherogram is a digital graph displaying a series of peaks. The position of each peak corresponds to the size of the STR fragment and the number of repeats at that locus. Analyzing the unique pattern of repeat numbers across all targeted STR loci generates a unique genetic profile, allowing forensic scientists to compare the sample to a known individual.
Forensic and Investigative Applications
DNA profiling is a powerful tool that has revolutionized forensic investigation by providing a scientifically reliable method of identification. A primary application is identifying suspects by matching DNA evidence from a crime scene with a known individual’s profile. Conversely, the technology can exonerate individuals who were wrongly convicted, providing definitive proof of innocence in post-conviction cases.
The analysis is also widely used in identifying victims of mass casualty events where traditional methods may not be feasible. Law enforcement utilizes DNA profiles to link multiple crime scenes to a single perpetrator, effectively identifying serial offenders. Beyond standard autosomal STR analysis, specialized techniques offer unique investigative advantages.
Specialized Techniques
- Y-chromosome (Y-DNA) analysis focuses on the Y chromosome, passed directly from father to son. This is useful in sexual assault cases to isolate male DNA from female DNA.
- Mitochondrial DNA (mtDNA) profiling is inherited solely through the maternal line and is present in high copy numbers. This is valuable for analyzing highly degraded samples like hair shafts, bones, or teeth where nuclear DNA is insufficient.
Data Interpretation and Quality Control
Once a DNA profile is generated, the next step involves statistical analysis and comparison against known profiles. The reliability of a match is expressed using the random match probability (RMP), which is the estimated chance that a random, unrelated individual would coincidentally have the identical DNA profile. For a full profile, this probability is often stated as one in many billions, conveying a high degree of certainty for individual identification.
Interpreting DNA mixtures, which contain genetic material from two or more contributors, presents a unique challenge. Factors like the number of contributors and the ratio of their DNA complicate the interpretation of the resulting electropherogram. Sophisticated statistical software, often employing probabilistic genotyping, is used to deconvolute these complex profiles and provide a likelihood ratio, a more informative measure of the evidence’s weight.
Profiles are stored and searched using national database systems, such as the CODIS in the United States. This database contains profiles from convicted offenders, arrestees, and unsolved crime scenes, allowing investigators to search for matches and link profiles from different crime scenes. Quality control is maintained through strict chain of custody protocols and rigorous laboratory standards to prevent sample contamination.