DNA profiling, often referred to as DNA fingerprinting, is a scientific technique used to identify individuals based on unique patterns in their genetic material. This process focuses on specific, highly variable sections of deoxyribonucleic acid (DNA) to create a distinctive genetic signature. The resulting DNA profile is a numerical representation that allows scientists to distinguish one person from another with a high degree of certainty.
The Science Behind DNA Profiling
Individual identification relies on the fact that while over 99.9% of human DNA is identical across all people, certain small regions exhibit significant variation. These variable segments, known as polymorphisms, are targeted by modern profiling techniques. The current standard method focuses on Short Tandem Repeats (STRs), which are short sequences of non-coding DNA, typically two to seven base pairs long, repeated multiple times.
The number of repeats at a specific location, or locus, on a chromosome differs between individuals. Since a person inherits one chromosome from each parent, they have two alleles (repeat counts) for every STR locus, forming a unique genotype, such as “10, 12.”
Analyzing multiple independent STR loci across the genome generates a unique profile. Standardized systems, like the FBI’s Combined DNA Index System (CODIS), examine a specific set of these markers, creating a genetic barcode statistically unique to every person, excluding identical twins.
The Steps of Creating a DNA Profile
Extraction
The creation of a DNA profile begins with Extraction, where DNA is isolated and purified from a biological sample such as blood, saliva, hair, or skin cells. This step separates the DNA from other cellular components and contaminants, preparing it for analysis. Even very small or degraded samples can yield enough material for the next stage.
Amplification
The second step is Amplification, performed using the Polymerase Chain Reaction (PCR). PCR acts as a molecular copier, making millions of copies of the targeted STR regions. Specific primers are used to bind to the DNA flanking the STRs, allowing a heat-stable enzyme to synthesize new strands repeatedly.
Separation and Detection
Next is Separation and Detection, typically carried out using capillary electrophoresis. The amplified DNA fragments, labeled with fluorescent dyes, are passed through a thin tube containing a gel-like material. An electrical current pulls the fragments through, separating them by size, with shorter fragments moving faster.
Analysis
Finally, the process concludes with Analysis, where a detector reads the color and size of each fragment, generating an electropherogram. This graph displays the size of the DNA fragments, which corresponds to the number of STR repeats at each locus, creating the complete numerical DNA profile.
Primary Applications of DNA Profiling
Forensics
DNA profiling is primarily used within the criminal justice system. In Forensics, profiles derived from crime scene evidence, such as blood or semen, are compared against profiles from suspects or forensic databases. This comparison can link a suspect to a crime scene or exclude an innocent person from an investigation.
Paternity and Kinship Testing
Beyond criminal cases, the technique is routinely used for Paternity and Kinship Testing. Since an individual inherits one allele for each STR locus from each biological parent, comparing profiles can establish biological relationships. This includes confirming parentage or determining familial ties for immigration purposes.
Mass Disaster and Missing Persons Identification
DNA profiling is also used to identify individuals involved in large-scale tragedies. Profiles are generated from unidentified remains and compared against reference samples provided by family members. This method is often the sole reliable means of identifying victims who are otherwise unrecognizable, providing closure and aiding in official documentation.
Understanding DNA Match Results
A DNA profile match is interpreted through statistical probability, not simple certainty. Data analysis calculates the likelihood of an unrelated individual randomly having the exact same combination of STR alleles, known as the Random Match Probability (RMP).
Forensic scientists calculate the RMP using the Product Rule, which involves multiplying the known population frequencies of each individual allele across all tested loci. Since STR loci are inherited independently, multiplying these probabilities results in a very small final number. For a full profile using the standardized marker set, the RMP often falls in the range of one in many billions or trillions, indicating the profile is essentially unique to one person (excluding identical twins).
When comparing two profiles, the result is either an Inclusion (profiles are statistically identical and cannot be excluded as originating from the same source) or an Exclusion (profiles are different at one or more loci and must come from different people). Law enforcement utilizes centralized databases, such as CODIS, to efficiently compare crime scene profiles against millions of known profiles from convicted offenders and arrestees.