Forensic DNA analysis uses genetic material found at a crime scene or on evidence to identify individuals, establish parentage, or investigate human remains. This method provides a highly reliable means of linking perpetrators to crimes or exonerating the wrongly accused. By transforming microscopic biological evidence into a distinctive genetic profile, DNA analysis provides investigators with objective leads. The process is highly standardized, ensuring results are consistent and effective in legal proceedings.
The Biological Foundation of Forensic Identification
DNA analysis distinguishes between individuals because no two people, aside from identical twins, possess the exact same genetic code. Deoxyribonucleic acid (DNA) is the biological blueprint found in nearly every cell of the human body, inherited equally from both parents. While the vast majority of human DNA is identical across the population, the remaining fraction contains unique variations.
Forensic science focuses on specific non-coding regions of the DNA, which do not contain instructions for making proteins and are highly variable without affecting biological function. The markers used are called Short Tandem Repeats (STRs), which are short sequences of two to six base pairs that are repeated multiple times. These variations in the number of repeats at multiple STR locations, or loci, form the basis of an individual’s unique DNA profile.
A modern forensic profile examines a panel of 20 or more autosomal STR loci, plus a marker for gender determination. Because the number of repeats for each STR location is inherited independently and is highly variable, the chance of two unrelated individuals sharing the same pattern across all tested loci is extremely small, providing the necessary statistical power for identification.
The Step-by-Step Laboratory Process
The transformation of a biological sample into a usable profile begins with the physical evidence. After a sample, such as a blood stain or saliva swab, is collected and preserved, the first analytical step in the laboratory is DNA extraction. Extraction separates the DNA from the other cellular components and contaminants present in the biological material.
Following extraction, quantification determines the amount of human DNA recovered from the sample. This measurement ensures there is sufficient material for the next stage and informs the subsequent amplification process. Samples containing very low amounts of DNA may require specialized, high-sensitivity protocols.
The third step is amplification, which uses a technique called Polymerase Chain Reaction (PCR). PCR creates millions of identical copies of the targeted STR regions of the DNA. This is necessary because crime scene samples often contain only minute or degraded amounts of DNA, and PCR generates enough material to create a detectable profile.
The final laboratory step involves separation and detection, typically accomplished using capillary electrophoresis. The amplified DNA fragments, chemically labeled with fluorescent tags during the PCR process, are passed through a fine glass capillary tube. This process separates the fragments by size, with shorter fragments traveling faster than longer ones.
As the fragments pass a laser detector, the instrument records the color and size of each fragment, corresponding to the number of repeats at each specific STR location. The resulting data, visualized as a graph of peaks called an electropherogram, constitutes the final DNA profile. This profile is a series of numbers representing the number of repeats at each tested locus, ready for comparison and interpretation.
Interpreting DNA Profiles and Matching
Once a DNA profile is generated from crime scene evidence, the next step is to compare it against known samples, or reference samples, from victims or suspects. A comparison involves checking if the number of repeats at every tested STR locus matches between the unknown and known profiles. If the profiles are identical at every point of comparison, it is declared a full match.
The reliability of a match is reported as a statistical probability, reflecting the scientific principle that two unrelated individuals could coincidentally share the same profile. This statistical analysis calculates the probability that a randomly selected, unrelated person would have the same DNA profile, resulting in a figure often less than one in a trillion. Profiles can also result in a partial match, where some but not all alleles align, or a mixture, which indicates the presence of DNA from two or more contributors.
If no suspect is immediately identified, the crime scene profile is searched against the Combined DNA Index System (CODIS). CODIS is the software that manages the National DNA Index System (NDIS), a centralized database. This database contains DNA profiles from convicted offenders, arrestees, missing persons, and unsolved cases. A match within CODIS, often called a “hit,” links an unknown profile to a person already in the system or connects two different crime scenes, providing investigative leads.
Quality control involves the analysis of control samples to ensure reagents are working correctly and to detect contamination. Contamination safeguards are a priority, as the sensitivity of modern testing means that even trace amounts of DNA from laboratory personnel or prior samples can be inadvertently amplified. These rigorous protocols ensure that the profiles used for comparison are accurate and reliable.
Expanding the Scope of Forensic DNA Use
Beyond the standard comparison of a crime scene sample to a known suspect, forensic DNA analysis has evolved to include several specialized applications that expand its investigative reach. One such technique is familial searching, which is used when a direct match to a crime scene profile is not found in the CODIS database. This involves searching the database for profiles that are very similar to the unknown sample, suggesting a close biological relationship, such as a parent, child, or sibling.
Familial searching provides law enforcement with a lead that can be used to identify a suspect through their relative, which is a powerful tool in solving cold cases. Another method is the analysis of Mitochondrial DNA (mtDNA), which is inherited solely from the mother and is present in high numbers in each cell. Because mtDNA is more robust than nuclear DNA, it is particularly useful for analyzing highly degraded biological evidence, such as old hair shafts, bones, or unidentified human remains.
The integration of forensic DNA with public genetic genealogy databases has created a novel tool known as forensic genetic genealogy (FGG). FGG uses different genetic markers than CODIS and searches consumer DNA databases to find distant relatives of the unknown profile. Genealogists then use this information to build family trees and identify potential suspects, a technique that has successfully resolved hundreds of cold cases.
Forensic DNA analysis is used for mass disaster victim identification. By comparing DNA profiles from remains to personal items or to samples provided by family members, the technology provides a definitive identification of those lost in large-scale tragedies.