When a DNA relationship test returns a result of “Not Excluded,” it means that the alleged parent cannot be scientifically ruled out as the biological parent of the child. The genetic profiles of the tested individuals are found to be compatible. The result signifies that the alleged parent possesses the necessary genetic material that the child inherited from one of their two biological parents. This initial determination then requires further statistical analysis to establish the conclusive certainty of the relationship.
Defining “Not Excluded” in Genetic Terms
The DNA comparison relies on analyzing specific regions of the genome called Short Tandem Repeats (STRs), which are short, repeating sequences of DNA bases found at various locations, or loci, on the chromosomes. Every person inherits two versions, or alleles, for each STR locus: one from the biological mother and one from the biological father. Testing typically involves comparing between 16 and 24 of these highly variable genetic markers across the tested individuals.
The child’s genetic profile is essentially a combination of the mother’s and the father’s profiles. For a non-exclusion result to be reached, at every single tested locus, the alleged father must possess at least one allele that matches the child’s profile and that the child did not inherit from the mother. If the mother is tested, her contribution is accounted for, isolating the genetic material that must have come from the father. The “Not Excluded” conclusion confirms that the alleged father’s genetic makeup is consistent with being the source of those paternal alleles.
This qualitative match demonstrates genetic compatibility, but it does not yet quantify the certainty of the relationship. Since millions of people share common genetic markers, a simple match is not enough to declare paternity definitively. The non-exclusion result simply means that no genetic evidence was found to contradict the claim of parentage, setting the stage for the statistical analysis that follows.
The Binary Outcomes: Exclusion Versus Non-Exclusion
DNA relationship tests yield one of two possible outcomes: exclusion or non-exclusion. An “Exclusion” is a definitive finding that the tested man is not the biological father of the child. This result occurs if there are mismatches at a sufficient number of STR loci, typically two or more, between the child’s obligate paternal alleles and the alleged father’s profile, ruling out paternity with 100% certainty.
The “Non-Exclusion” result means that the tested man cannot be ruled out as the father because a genetic link is supported by the shared alleles. Unlike exclusion, non-exclusion is not a statement of absolute fact on its own. Therefore, the non-exclusion finding must be mathematically evaluated to determine the statistical weight of the match and to provide a measure of confidence. This crucial difference is why the high certainty percentages are attached to a non-exclusion result, transforming a compatible match into a legally and scientifically confirmed relationship.
Understanding the Paternity Index and Probability
The certainty of a non-exclusion result is quantified by two related statistical values: the Paternity Index (PI) and the Probability of Paternity (POP). The PI is a likelihood ratio calculated for each individual genetic marker, comparing the likelihood that the tested man passed on the necessary alleles to the child, versus the likelihood that a random, untested man from the same population group could have passed on those same alleles.
The individual PIs from all tested loci are then multiplied together to calculate the Combined Paternity Index (CPI). A very high CPI indicates that the tested man is many millions of times more likely to be the biological father than a random man. This CPI value is then converted into the Probability of Paternity (POP), which is the final, easy-to-understand percentage presented on the report.
Laboratories use a standard prior probability of 0.5, representing a neutral starting point that the tested man is equally likely or unlikely to be the father before the genetic data is considered. When the CPI is high enough, the POP rounds to \(99.9\%\) or higher, which is the standard threshold for a conclusive result. For instance, a result of \(99.99\%\) means that the evidence overwhelmingly supports the conclusion that the tested man is the biological father.
Practical Implications of a Non-Exclusion Result
A non-exclusion result with a high probability of paternity is the final documentation required to establish a biological relationship for various purposes. Testing is categorized into two types: informational and legal testing. Informational tests are for personal use and can be collected at home, but they are not admissible in court because they lack strict oversight.
Legal DNA testing is required for official proceedings and follows a rigorous “chain of custody” protocol, where sample collection is witnessed by a certified third party and identity is verified. Legal results are used in child support and custody disputes, immigration cases, and for amending official documents like birth certificates. A non-exclusion result with a probability of paternity of \(99.9\%\) or higher is considered scientifically and legally definitive proof of parentage.