A paternity test is a highly accurate scientific method used to establish the biological relationship between a child and an alleged father. This process relies on analyzing DNA samples, most commonly collected through a simple, non-invasive cheek swab. The test compares specific segments of genetic material to determine if the child inherited their DNA from the man being tested, providing definitive evidence for legal, medical, or personal reasons.
The Foundation of Genetic Inheritance
The process hinges on the fundamental biological rule of genetic inheritance. Every human cell contains DNA, organized into 23 pairs of chromosomes. A child inherits one complete set of 23 chromosomes from their biological mother and one complete set from their biological father.
This ensures the child receives approximately 50% of their total genetic material from each parent, making the child’s genetic profile a precise combination of the parents’ profiles.
Inheritance follows Mendelian principles, meaning the child receives one specific version of a genetic location, known as an allele, from each parent. Paternity testing verifies that the alleles the child inherited from the father are consistent with the alleged father’s own genetic profile.
Targeting DNA Markers for Comparison
The test does not analyze the entire genome. Instead, scientists focus on specific, highly variable regions called Short Tandem Repeats (STRs). STRs are short sequences of two to six base pairs repeated multiple times in non-coding DNA regions.
The number of times a sequence is repeated varies significantly between individuals, making STRs useful for identification. These different repeat numbers constitute the alleles that are inherited and compared.
Laboratories analyze a standardized set of 16 to 25 different STR locations, or loci, across the chromosomes. The combination of alleles across all these locations creates a unique genetic fingerprint. By comparing the child’s STR alleles with the alleged father’s set, the test determines a biological link with high certainty.
Laboratory Steps: Amplification and Profiling
The testing begins with collecting a biological sample, usually a buccal swab from the inside of the cheek. This swab contains epithelial cells, from which the DNA is extracted in the laboratory. The purified DNA is then subjected to Polymerase Chain Reaction (PCR).
PCR is a technique used to exponentially amplify, or copy, the specific STR regions targeted for analysis. The process uses a thermal cycler to repeatedly heat and cool the DNA sample in the presence of primers. This cycling creates millions of copies of the targeted STR fragments, making them detectable for the next step.
After amplification, the DNA fragments are separated and measured using Capillary Electrophoresis. The amplified, fluorescently labeled fragments are drawn through thin glass tubes containing a gel-like material by an electric current. Shorter fragments travel faster, separating them precisely by size.
A laser detects the fluorescent label on each fragment, and the data is translated into an electropherogram. This graph displays peaks where the position corresponds to the fragment’s size. The size of the fragment correlates directly to the number of repeats at that STR location, providing the two allele numbers for the individual’s genetic profile.
Interpreting the Results
The final step involves comparing the genetic profiles generated for the child, the mother (if tested), and the alleged father. At every tested STR location, the child’s two alleles must be traceable back to one allele from the mother and one allele from the alleged father.
If the child possesses an allele absent in both the mother and the alleged father, the alleged father is excluded from paternity.
If the profiles match across all tested markers, a statistical calculation determines the strength of the genetic evidence. This calculation produces the Probability of Paternity (POP). The POP is derived from the Combined Paternity Index, which compares the likelihood of the alleged father passing on the necessary genetic markers versus a random, untested man in the population.
For the alleged father to be considered the biological parent, the Probability of Paternity must meet a threshold of 99.9% or higher. Since the calculation is based on statistical probabilities and population frequencies, the result is never reported as 100%. A result of 0% probability is reported if two or more mismatches occur, conclusively excluding the alleged father.