Deoxyribonucleic acid, or DNA, is the blueprint for life. Each individual’s DNA sequence is largely unique, making it a powerful identifier. This distinctiveness allows DNA to act as a biological signature, which forensic scientists utilize in investigations. Its stability and presence in nearly every cell make it an invaluable tool for linking individuals to crime scenes or specific events.
Collecting and Preserving DNA Evidence
Collecting DNA evidence at a crime scene requires careful handling to prevent contamination and ensure integrity. Common sources of DNA include blood, saliva, semen, hair with roots, skin cells, and sweat. Forensic technicians use sterile gloves and tools to collect items that may contain biological material, such as discarded cups, cigarette butts, or clothing. Each item is packaged separately to avoid cross-contamination.
Once collected, proper preservation is crucial to maintain DNA quality. Samples are dried if wet, as moisture promotes bacterial growth that degrades DNA. After drying, evidence is stored in paper bags or envelopes to allow air circulation and kept in cool, dry conditions, such as refrigeration. This ensures the DNA remains suitable for analysis, preserving the chain of custody.
Analyzing DNA in the Lab
Once DNA evidence arrives at the laboratory, forensic scientists begin a multi-step process to generate a usable DNA profile. The first step involves extracting DNA from the collected biological material, separating it from other cellular components. Following extraction, the quantity of human DNA in the sample is measured to determine if there is enough material for further analysis. This step, known as quantification, helps optimize subsequent procedures.
After quantification, a technique called Polymerase Chain Reaction (PCR) is used to amplify specific regions of the DNA. PCR creates millions of copies of targeted DNA segments, even from very small or degraded samples, making them detectable. Forensic DNA analysis primarily focuses on Short Tandem Repeats (STRs), which are short, repeating DNA sequences that vary significantly in length between individuals. Scientists analyze multiple STR regions, typically between 13 and 20 specific loci, to create a unique numerical profile.
These amplified STR fragments are then separated by size using a method like capillary electrophoresis, which produces an electropherogram—a graphical representation of the DNA profile. Each peak on the electropherogram corresponds to a specific STR allele, and the combination of these alleles across multiple loci forms a distinctive DNA profile. This profile acts as a unique genetic fingerprint, allowing for individual identification.
Matching DNA Profiles and Databases
After a DNA profile is generated from crime scene evidence, it can be compared to profiles from known suspects or against vast databases. In the United States, the primary database used for this purpose is the Combined DNA Index System (CODIS), managed by the Federal Bureau of Investigation (FBI). CODIS contains DNA profiles from convicted offenders, arrestees, unsolved crime scenes, and missing persons, enabling law enforcement to link cases across jurisdictions.
When a crime scene profile is uploaded to CODIS, the system performs an automated search for matches within its various indices. A “hit” indicates a potential match, which is then verified by forensic scientists. The power of DNA evidence lies in the statistical probability of a random match. For a full DNA profile derived from 13 or more STR loci, the probability of two unrelated individuals having the same profile is exceedingly low, often less than one in a quadrillion.
This low probability means a DNA match provides strong evidence for identification. DNA profiles can link a suspect to a crime, connect multiple crime scenes, or identify human remains. The systematic comparison through databases like CODIS has transformed forensic investigations, allowing for the re-examination of cold cases and the identification of previously unknown offenders.
What DNA Evidence Can and Cannot Tell Us
DNA evidence can provide definitive answers regarding identification, exclusion, and familial relationships. It can identify an individual, confirm a person’s presence at a crime scene, or exclude a suspect. DNA can also establish paternity or maternity, and link siblings or other relatives in cases of missing persons or unidentified remains. Its ability to pinpoint identity with high statistical certainty makes it a valuable investigative tool.
However, DNA evidence also has limitations and requires careful interpretation within the broader context of an investigation. Degraded samples, often found at older crime scenes or those exposed to environmental elements, may yield incomplete DNA profiles. Mixed samples, containing DNA from two or more individuals, can also complicate analysis, making it challenging to decipher individual contributions. The presence of DNA at a crime scene does not automatically imply guilt.
Contamination remains a concern, as foreign DNA can be inadvertently introduced to a sample during collection or analysis. Secondary transfer, where DNA moves indirectly from one person to an object and then to a crime scene, means a person’s DNA can be present without them being physically at the location. Therefore, while DNA offers strong scientific data, it is one piece of the puzzle and must be considered alongside other evidence to reconstruct events accurately.