DNA fingerprinting, more accurately termed DNA profiling, is a laboratory technique used to distinguish individuals based on variations in their genetic code. The core principle involves analyzing specific segments of deoxyribonucleic acid (DNA) that differ significantly from one person to the next. Since its development by British geneticist Sir Alec Jeffreys in the mid-1980s, this method has fundamentally changed fields ranging from criminal justice to medical diagnostics.
The Biological Basis of Identity
The vast majority of human DNA, approximately 99.9%, is identical across all people. DNA profiling focuses on the small fraction of the genome where sequence patterns show considerable variability, primarily non-coding segments known as Short Tandem Repeats (STRs).
STRs are short sequences of DNA, typically two to seven base pairs long, repeated multiple times at specific locations, or loci, on a chromosome. The number of repeats varies greatly among individuals, a phenomenon known as polymorphism. For example, one person might have ten repeats of a sequence at a given locus, while another might have thirteen.
Since a person inherits one set of chromosomes from each parent, they possess two alleles at every STR locus. These two repeat numbers constitute their unique genetic marker at that site. By analyzing multiple STR loci simultaneously, scientists generate a genetic signature unique to nearly every individual, except identical twins. The probability of two unrelated people sharing the same complete profile is extremely remote.
Steps in Creating a DNA Profile
The process of creating a DNA profile begins with collecting a biological sample, such as blood, saliva, or skin cells, followed by DNA extraction. The goal of extraction is to isolate the DNA from other cellular components using chemical methods. The amount of usable DNA recovered can be minute, especially from aged or degraded samples found at a crime scene.
Once the DNA is extracted, the next step is amplification using the Polymerase Chain Reaction (PCR). PCR rapidly and selectively replicates only the targeted STR regions of the DNA, even if the starting material is extremely limited. This process uses specific short DNA primers that flank the target STR regions, along with a heat-stable DNA polymerase enzyme. Through repeated cycles of heating and cooling, millions of copies of the chosen STR segments are generated.
The amplified DNA fragments are then separated and measured using capillary electrophoresis. The DNA fragments, which are tagged with fluorescent dyes, are passed through a fine glass tube containing a polymer solution. An electric current pulls the fragments through the tube, separating them based on size, with shorter fragments traveling faster than longer ones. A laser beam detects the fluorescent tag on each fragment, and the data is recorded as an electropherogram.
The final step involves interpreting the data, which translates the fragment sizes into a numerical profile. Each peak on the electropherogram corresponds to an allele at a specific STR locus, and the fragment size indicates the exact number of repeats. The resulting profile is a series of numbers representing the alleles at the analyzed loci, which can then be compared against other samples or entered into a database.
Primary Uses in Legal and Medical Fields
DNA profiling is a standard tool in forensic science, used to link individuals to evidence recovered from a crime scene. By comparing the genetic profile from a sample of unknown origin, such as semen or blood, with that of a suspect, investigators can generate high probability statistics regarding a match. This technology is also important for exonerating individuals who have been wrongfully convicted, providing proof of innocence in post-conviction cases.
DNA profiling is also widely utilized in establishing biological relationships. Paternity testing involves comparing the STR profiles of the child, mother, and alleged father. Since a child inherits half of their alleles from each parent, a conclusive match or exclusion can be made based on the shared genetic markers.
The technique also serves a humanitarian role in identifying human remains, particularly in mass casualty events. By comparing the genetic profile of unidentified remains to reference samples from relatives, scientists can provide closure to families. In the medical field, STR analysis methods are used to monitor the success of bone marrow transplantation by assessing the engraftment of donor cells in the recipient.
Data Storage and Ethical Considerations
The utility of DNA profiling is significantly amplified by the use of national databases, such as the Combined DNA Index System (CODIS) in the United States. These centralized repositories store millions of standardized genetic profiles collected from convicted offenders, arrestees, and crime scene evidence. The primary function of these systems is to enable law enforcement agencies across jurisdictions to search for matches, generating investigative leads.
The existence and expansion of these genetic databases introduce several complex ethical and privacy concerns. A central debate revolves around the scope of collection, particularly when DNA is taken from individuals who have only been arrested and not convicted, raising questions about civil liberties. The potential for misuse of this information, which can reveal medical details and familial connections, underscores the need for stringent security protocols and clear legal guidelines.
Furthermore, the practice of familial searching, which looks for partial matches to identify close relatives of a suspect, raises concerns about the privacy of family members. Maintaining the accuracy and integrity of the data is essential, as contamination or error in sample processing can lead to miscarriages of justice. The governance of these databases requires a continuous balance between public safety interests and the protection of individual genetic privacy.