Forensic DNA analysis uses an individual’s unique genetic blueprint, deoxyribonucleic acid (DNA), found in biological evidence to answer legal questions. This scientific discipline focuses on identification, determining whether a known person can be included or excluded as the source of biological material collected from a crime scene or other context. Because every person’s DNA sequence is distinct, forensic scientists create a genetic profile that acts like a highly specific identifier. The successful application of this technology relies on a standardized laboratory process and statistical interpretation.
The Scientific Methodology
The path from a biological sample collected at a scene to a usable genetic profile involves a defined sequence of laboratory procedures. The first step, DNA extraction, isolates the genetic material from the cells of the evidence sample, such as blood, saliva, or skin cells. Following extraction, quantitation measures the precise amount of human DNA recovered to ensure enough material exists for analysis.
The process then moves to amplification, using Polymerase Chain Reaction (PCR) to create millions of copies of specific DNA regions. PCR is necessary because forensic samples often contain only minute amounts of DNA. The regions targeted are Short Tandem Repeats (STRs), which are short sequences of DNA that repeat multiple times at specific locations, or loci, across the chromosomes.
The power of STR analysis lies in the variability of these repeats, as the number of repetitions at each locus differs significantly between individuals. Scientists analyze multiple STR loci—often 16 to 22 regions—to build a comprehensive profile. The final step, separation and analysis, involves running the amplified fragments through a process like capillary electrophoresis, which separates the STR fragments by size. The resulting data, displayed as a series of peaks on an electropherogram, represents the unique DNA profile that can be compared to known reference samples.
Key Applications in Criminal Justice
Once a DNA profile is generated from a forensic sample, its primary function is to serve as an investigative tool and source of evidence. The most common application involves linking a suspect to a crime scene by comparing their known profile to the evidence profile. A match provides strong evidence of an association between the individual and the location where the biological material was left.
DNA analysis is used in several key areas:
- Linking suspects to crime scenes.
- Solving “cold cases” by re-examining old evidence with modern techniques.
- Exoneration, proving that a wrongly convicted person did not commit the crime.
- Human identification, such as identifying victims of mass casualty incidents or unidentified human remains by comparing profiles to known relatives.
National DNA Databases
The systematic comparison of DNA profiles is made possible through national databases that store standardized genetic information. The primary example in the United States is the Combined DNA Index System (CODIS), maintained by the Federal Bureau of Investigation (FBI). This system operates on three tiers—local, state, and national—allowing for the electronic sharing and comparison of profiles across jurisdictions.
CODIS stores standardized STR profiles and contains indexes for convicted offenders, arrestees, and forensic evidence from unsolved cases. When a crime scene profile matches a profile already in the database, it is known as a “cold hit.” This functionality allows investigators to link crimes across different geographic areas or identify potential suspects. For privacy, the database stores only the numerical STR profile and case information, not personal identifying details like names or medical history.
Understanding Reliability and Limitations
While forensic DNA evidence is powerful, its interpretation is subject to scientific and procedural limitations.
Contamination and Mixed Samples
Contamination is a persistent risk, where foreign DNA from investigators, laboratory personnel, or other evidence inadvertently mixes with the sample being tested. Because modern techniques can amplify minute amounts of DNA, even trace amounts can cause a mixed profile, potentially leading to misleading results.
The analysis of mixed samples, which contain DNA from two or more individuals, presents a significant challenge. Deconvoluting the separate profiles often requires complex software and subjective interpretation.
Degradation
Another limitation is sample degradation, where environmental factors like heat, moisture, or bacterial activity break down the long DNA molecules into smaller fragments. If the DNA is severely degraded, it may fail to amplify during the PCR process, preventing the generation of a complete profile.
Statistical Interpretation
When a match is declared, its meaning is quantified using a statistic called the random match probability (RMP). The RMP is the calculated likelihood that a random, unrelated person in a given population would possess the exact same genetic profile as the one found in the evidence. This probability is determined by multiplying the frequencies of the individual STR alleles across all tested loci. This typically results in an extremely small number, often in the range of one in trillions, defining the strength of the evidence rather than claiming absolute certainty.