The analogy of a “genetic fingerprint” is powerful because, like the unique patterns on a person’s hand, DNA provides an individual molecular signature. While actual fingerprints are visible skin patterns, the genetic version is a distinct arrangement of molecules within the body’s cells. This molecular signature allows for precise identification and differentiation among individuals. Every person, with the exception of identical twins, possesses a wholly unique genetic code.
The biological foundation for individual identity rests on the variability of the human genome. Although over 99% of the DNA sequence is identical across all humans, the remaining fraction contains enough differences to distinguish one person from another. Scientists focus identification efforts on these highly variable regions of the genetic code. These differentiating sequences are typically found in non-coding DNA, outside of the genes that code for proteins.
The primary markers used for generating a genetic profile are Short Tandem Repeats (STRs), which are sequences of two to six base pairs that are repeated multiple times in a row. The variability that makes STRs so useful comes from the number of times this short sequence is repeated at a specific location, or locus, on a chromosome. For example, at one locus, a person might have 10 repeats, while another person might have 12 repeats. Since one copy of the DNA is inherited from each parent, an individual has two alleles, or versions, for each STR locus, which combine to create a unique pair of numbers.
This characteristic of STRs creates a highly discriminating genetic marker. When multiple STR loci are analyzed together, the probability of two unrelated individuals sharing the exact same combined profile becomes extremely low. These genetic markers are stable, meaning they do not change throughout a person’s life, and they follow predictable patterns of inheritance. Analyzing these inherited and highly variable STR regions provides the biological material for constructing a unique genetic profile.
Translating DNA into a Unique Profile
Creating a usable genetic profile requires a specific, multi-step laboratory methodology. The process begins with collecting a biological sample, such as a cheek swab, blood, or saliva, followed by extracting the DNA from the cells. Once isolated, the quantity and quality of the DNA extract are measured to ensure optimal conditions for the next steps.
The next step is amplification, which utilizes a technique called Polymerase Chain Reaction (PCR) to rapidly create millions of identical copies of the targeted STR regions. PCR cycles through heating and cooling to separate the DNA strands, using specialized enzymes to synthesize new strands and copy the specific STR sequences. This amplification is necessary because biological samples often contain only minute amounts of DNA.
After amplification, the copied fragments are separated based on their length, a step commonly performed using capillary electrophoresis. The STR fragments differ in length due to the varying number of repeats and are pulled through a thin tube by an electric current. Shorter fragments travel faster than longer ones. As the fragments pass a detector, a laser excites fluorescent tags attached to the DNA, and the resulting signal is read by a camera.
The output of the separation process is an electropherogram, which is a graph showing peaks corresponding to the size and quantity of each DNA fragment. Each peak represents an allele at a specific STR locus, and the fragment size is translated into the number of repeats present. The profile is ultimately converted into a series of numbers that represent the two alleles at each of the standardized STR loci analyzed.
These numerical profiles are then stored and compared against other profiles in secure, standardized databases, such as the Combined DNA Index System (CODIS) used in the United States. Standardizing the set of STR loci analyzed allows laboratories to electronically exchange and compare genetic information. This comparison allows a sample profile to be matched against known profiles or against profiles from unsolved cases.
Essential Applications of Genetic Identification
The ability to translate a biological sample into a unique numerical profile has transformed several fields by providing an accurate method of identification. One primary use is forensic identification, where the genetic profile acts as an investigative tool. Profiles generated from evidence left at a crime scene, such as hair, blood, or skin cells, can be compared to profiles of suspects. A match strongly links an individual to the crime scene, while an exclusion can help exonerate an innocent person.
Genetic profiles are also routinely used for establishing biological relationships, most commonly in paternity and kinship testing. Because a child inherits one allele at each STR locus from each biological parent, a child’s profile must be a combination of the parents’ profiles. This predictable inheritance pattern allows for the establishment of paternity with a high degree of certainty, often exceeding 99.9% probability. This type of testing is important in legal matters such as child support and inheritance claims.
A third significant application is the identification of human remains, particularly in cases of mass disasters or unidentified bodies. DNA profiles are generated from the remains and then compared to profiles obtained from personal effects or relatives of missing persons. This process provides a reliable method for disaster victim identification, offering certainty where traditional methods may be impossible due to the condition of the remains.