DNA comparison examines genetic material to determine the degree of similarity or difference between two or more DNA samples. By identifying shared genetic markers, it reveals connections between people, families, or populations. This method is a foundational technology across modern scientific disciplines.
The Science of Genetic Matching
Human DNA is remarkably similar across individuals, with approximately 99.9% of the genetic code being identical. The remaining fraction contains highly variable regions that allow for individual distinction.
Among these variable regions are Short Tandem Repeats (STRs). These are short sequences of DNA, typically 2 to 7 base pairs long, repeated multiple times. The number of repeats varies significantly from person to person, forming a unique genetic signature.
To analyze these minute STR regions, Polymerase Chain Reaction (PCR) is used. PCR is a laboratory method that rapidly produces millions to billions of copies of a specific DNA segment, which is necessary because initial samples are often very small.
The PCR process involves several cycles of temperature changes. First, DNA strands are separated by heating the sample to about 95 degrees Celsius. Next, the temperature is lowered to around 55 degrees Celsius, allowing short synthetic DNA fragments called primers to attach to the specific STR regions. Finally, the temperature is raised to about 72 degrees Celsius, and a heat-stable enzyme, DNA polymerase, adds new nucleotides to build complementary DNA strands, copying the targeted STR regions. This cycle is repeated many times, exponentially increasing the amount of DNA.
Applications of DNA Comparison
DNA comparison is widely used across many fields, providing insights into relationships, identities, and historical connections.
Forensic Science
In forensic science, DNA comparison is used in criminal investigations. DNA samples collected from crime scenes, such as blood, saliva, or skin cells, can be compared to DNA from suspects. This comparison determines if a suspect’s genetic profile matches the evidence, linking them to the crime.
This method also assists in identifying human remains, particularly when visual identification is not possible due to decomposition or severe injury. DNA from the remains can be compared to samples from presumed family members to establish identity.
Paternity and Family Relationships
DNA comparison is extensively used to establish biological relationships, most commonly in paternity testing. By comparing the DNA of a child, mother, and alleged father, laboratories can determine with high certainty whether a biological father-child relationship exists, based on the inheritance patterns of genetic markers.
Beyond paternity, DNA comparison can clarify other family relationships, such as grandparentage, sibling relationships, or more distant familial ties. These tests help individuals confirm biological connections for personal reasons, legal matters, or inheritance claims.
Genealogy and Ancestry
Commercial DNA testing services help individuals explore their ethnic heritage and connect with unknown relatives. These services compare an individual’s DNA profile to large databases of genetic information from diverse populations worldwide, revealing ancestral origins and migration paths.
Individuals can discover distant cousins by identifying shared DNA segments with others in the company’s database. The length and number of shared segments, measured in centimorgans (cM), indicate the closeness of the relationship. Larger shared segments suggest a more recent common ancestor.
Interpreting Results and Accuracy
The results of DNA comparison are expressed as a statistical probability rather than an absolute “yes” or “no” answer. For instance, in paternity testing, a result might indicate a 99.9% probability of paternity, meaning the likelihood of another unrelated individual having that exact genetic profile is extremely low. This accounts for the inherent variability in human DNA.
Several factors influence the accuracy and reliability of DNA comparison results. The quality and integrity of the DNA sample are important; degraded, contaminated, or very small samples can yield less reliable results. Environmental factors like heat, humidity, or exposure to chemicals can damage DNA over time.
The number of genetic markers, specifically STR loci, analyzed, affects the discriminatory power of the test. Analyzing more markers increases the uniqueness of the genetic profile and the statistical certainty of a match. Laboratories use a standardized set of markers to ensure consistency and comparability of results.
Human error during sample collection, processing, or analysis can also affect outcomes. Strict laboratory protocols and quality control measures are in place to minimize such errors.