DNA profiling, also known as DNA fingerprinting, is a powerful laboratory technique used to distinguish one individual from another based on their unique genetic code. This method analyzes specific, highly variable regions within a person’s deoxyribonucleic acid (DNA) to create a distinctive genetic signature. It is especially useful in forensic science, providing a powerful tool for linking individuals to biological evidence. It relies on the principle that, with the exception of identical twins, no two people share the exact same genetic sequence.
The Unique Markers in Our Genome
The ability to differentiate individuals genetically stems from polymorphisms, which are variations in the DNA sequence that occur throughout the human genome. While nearly 99.9% of the human genome is identical across all people, the remaining fraction contains sufficient variation to create a unique identifier. The specific markers used for DNA profiling are not the genes that code for physical traits but rather regions of non-coding DNA.
The focus of modern DNA profiling is on specific regions called Short Tandem Repeats (STRs). These are short sequences of nucleotides, typically two to seven base pairs in length, that are repeated multiple times in a row at specific locations, or loci, on a chromosome.
The number of times an STR sequence repeats is highly variable among unrelated individuals, giving rise to the polymorphism that is measured. Since one copy of a chromosome is inherited from each parent, every person possesses two alleles—or versions—for each STR locus, one from each parent. The combination of the repeat numbers across multiple analyzed STR loci creates a genetic profile with statistical power strong enough to be virtually unique to an individual.
Extracting and Analyzing the Profile
The process of creating a DNA profile begins with the collection of a biological sample, which can be as small as a cheek swab, a drop of blood, or a few cells from a hair root or saliva. Once the sample is collected, the DNA must first be extracted from the cells using chemical methods to isolate the genetic material from other cellular components.
The isolated DNA is often present in minute quantities, especially at a crime scene, which necessitates a massive amplification step using the Polymerase Chain Reaction (PCR) technique. PCR rapidly creates billions of copies of the target STR regions, making the small sample large enough to be reliably analyzed. This process cycles through three temperatures: heating to separate the DNA strands (denaturation), cooling to allow short DNA primers to attach (annealing), and warming to allow the DNA polymerase enzyme to synthesize new strands (extension).
After amplification, the resulting DNA fragments are separated using a method like capillary electrophoresis. In this technique, an electrical current pulls the negatively charged DNA fragments through a thin tube. Shorter fragments travel faster and farther than longer fragments, separating them by size. A laser detects a fluorescent tag attached to the fragments, and the results are displayed as an electropherogram, a series of colored peaks that represent the size and quantity of the alleles present at each STR locus.
Identification in Legal Contexts
DNA profiles are used in the legal system for both criminal and civil matters. In forensic science, a profile generated from evidence found at a crime scene, such as semen, hair, or skin cells, can be compared directly to the profile of a suspect. A complete match provides compelling evidence of an individual’s presence at the scene, while an exclusion can exonerate a suspect.
DNA profiling is also routinely used to establish biological relationships in cases of kinship and paternity testing. Since a child inherits half of their genetic markers from each parent, their profile must contain a combination of the alleles found in both the mother and the father. By comparing the STR profiles of a child, mother, and alleged father, laboratories can calculate the statistical probability of a biological link. This application extends to identifying victims in mass disasters or human remains that are too degraded for traditional identification methods.
Reliability and Database Management
Reliability
While DNA profiling is considered a reliable form of identification, the process is subject to potential sources of error. Sample contamination, where foreign DNA is accidentally introduced, or degradation due to environmental factors can complicate the analysis or lead to misinterpretation. Forensic laboratories employ procedural safeguards, such as rigorous quality control and blind testing, to minimize the risk of human error.
Database Management
To maximize investigative power, governments maintain large genetic databases, such as the Combined DNA Index System (CODIS) in the United States. These databases store the profiles of convicted offenders, arrestees, and unidentified crime scene evidence, allowing law enforcement to search for matches across jurisdictions. The maintenance of these extensive databases introduces significant ethical and privacy concerns. Storing a person’s genetic blueprint raises questions about the long-term security of this sensitive personal information and who should have access to it, particularly if the profile was obtained from an individual who was never convicted of a crime.