A paternity test is a scientific process that uses genetic information to determine if a man is the biological father of a child. This DNA-based analysis compares the unique genetic profiles of individuals to establish a biological relationship. The procedure relies on the established principles of genetic inheritance to provide an accurate conclusion.
The Foundation: DNA and Genetic Markers
Every person inherits half of their genetic material from their biological mother and half from their biological father. This rule of inheritance is the basis for paternity testing. DNA testing focuses on specific, highly variable regions called Short Tandem Repeats (STRs), rather than the entire human genome.
STRs are sections of non-coding DNA where a short sequence of nucleotides is repeated multiple times. These repeat lengths vary significantly between unrelated people, making them excellent markers for individual identification. At each STR location, an individual possesses two versions, known as alleles, one inherited from each parent.
A standard paternity test analyzes between 16 and 25 of these specific STR locations. By comparing the alleles present at these sites, scientists determine the likelihood of a biological link. The child’s alleles at every tested location must match one allele from the mother and one allele from the alleged father.
Sample Collection and Preparation
The process begins with collecting a sample containing the individual’s DNA. The most common collection method uses a buccal swab, which is gently rubbed against the inside of the cheek. This collects epithelial cells that contain the person’s complete genetic code.
Individuals are asked to refrain from eating, drinking, or smoking before collection to ensure a high-quality sample. The swabs are air-dried completely to prevent DNA degradation during transport. For legal tests, a Chain of Custody protocol is implemented to guarantee sample integrity and prevent tampering. This involves a third-party collector verifying identities and sealing the samples before shipment.
Laboratory Analysis and Profile Comparison
Once samples arrive, cells are chemically treated for DNA extraction, isolating the genetic material. The extracted DNA is measured and then subjected to Polymerase Chain Reaction (PCR), a molecular photocopying technique.
PCR uses specific primers to rapidly copy the chosen STR regions millions of times. This amplification is necessary because the initial DNA sample is often minute, ensuring reliable analysis. The amplified DNA fragments are then separated by size using capillary electrophoresis (CE).
During CE, an electric current pulls the DNA fragments through thin glass tubes containing a polymer. Smaller fragments travel faster, separating the STR alleles based on their exact length. A laser detects fluorescent tags attached to the fragments, generating a unique genetic profile visualized as an electropherogram.
Interpreting the Results
The genetic profiles of the child, the mother (if tested), and the alleged father are compared side-by-side. For every STR marker, the child must possess one allele matching the mother and one matching the alleged father. If the child has an allele that cannot be attributed to the alleged father at two or more markers, the tested man is scientifically ruled out, resulting in an “Exclusion.” When all the child’s paternal alleles match the alleged father, the result is reported as “Not Excluded.” This match leads to a statistical calculation called the Probability of Paternity, which is almost always reported as a percentage greater than 99.99%.