Paternity testing establishes a biological relationship between a child and a potential father by analyzing and comparing their unique genetic blueprints. This method relies on the concept that a child inherits half of their genetic material from each biological parent. DNA test results are often presented visually through a gel electrophoresis image. This visual representation converts complex genetic data into a pattern that allows for the direct comparison and determination of parentage.
Visual Components of the Gel
The physical result of this test is a gel, a slab of material that separates DNA fragments. Each column, known as a lane, holds the DNA samples from the mother, the child, and the man being tested. Since DNA is negatively charged, applying an electric current causes the fragments to travel toward the positive end of the gel, a process called electrophoresis.
The gel acts like a sieve, allowing smaller DNA fragments to travel faster and farther down the lane, while larger fragments remain closer to the starting point. The visual lines in each lane are called bands, representing specific fragments of DNA (alleles) separated by size. Each band is a collection of identical DNA fragments that stopped at the same point.
One lane is dedicated to a molecular weight ladder, which contains DNA fragments of known sizes. This ladder serves as a reference scale, allowing scientists to determine the exact size of the fragments in the sample lanes. The final pattern of bands in each lane is unique to the individual, representing a distinct genetic profile.
Comparing Band Patterns for Paternity
Interpretation begins with the principle that a child receives one copy of genetic information from each parent at every tested location. Scientists analyze specific, highly variable regions of the DNA, known as short tandem repeats. For each tested region, the child’s genetic profile shows two bands, representing the two inherited alleles.
The first step is to identify the bands the child inherited from the mother. By aligning the mother’s lane with the child’s lane, one accounts for the bands matching the mother’s profile. The remaining band or bands in the child’s profile must have been inherited from the biological father.
The comparison then shifts to the alleged father’s lane to determine if his profile contains the necessary remaining bands. If the alleged father possesses a band corresponding in size and position to the child’s unmatched band, that marker is considered a match. This comparison is repeated across all tested regions of DNA to build a complete picture of inheritance.
Determining Inclusion or Exclusion
The final conclusion is determined by the cumulative results of band comparisons across all tested genetic regions. The result is either an Inclusion or an Exclusion. An Inclusion occurs when the alleged father’s DNA profile contains all the required paternal bands to complete the child’s profile at every tested location.
This outcome means the tested man cannot be ruled out as the biological father because his genetic information is consistent with the child’s inheritance. Conversely, an Exclusion is determined if a mismatch is found at one or more tested genetic locations. A mismatch means the alleged father lacks a band required to account for the child’s profile.
Even a single mismatch across the tested regions is enough to rule out the man as the biological father. The science requires that the child’s entire profile be explained by the combination of the mother’s and the father’s genetic information. If the comparison shows the child possesses a band that neither the mother nor the alleged father has, the tested man is excluded from paternity.
Understanding Statistical Certainty
While the visual reading provides the initial determination, the final report includes numerical metrics to quantify the certainty of the result. These statistics are applied when an inclusion is found to measure the strength of the genetic evidence. The Paternity Index (PI) represents how many times more likely it is that the tested man is the father compared to a random, unrelated man.
The PI is calculated for each tested genetic region; higher numbers indicate a stronger match based on the rarity of shared bands. Individual Paternity Index values are then multiplied together to generate the Combined Paternity Index (CPI). The CPI is a single number that summarizes the total genetic evidence across all markers.
The CPI is used to calculate the final and most commonly cited metric, the Probability of Paternity (POP), expressed as a percentage. This percentage represents the confidence level that the tested man is the biological father. For a result to be legally recognized as an inclusion, the POP is typically reported as 99.9% or higher, confirming that the genetic evidence supports the conclusion of biological fatherhood.