A DNA test report that concludes an individual “cannot be excluded” as the biological parent is a common source of confusion for those unfamiliar with genetic terminology. DNA testing is a robust tool used to establish biological relationships, particularly paternity, by comparing the genetic profiles of individuals. The specific language used in these reports reflects the statistical nature of the underlying scientific analysis. Understanding this phrase requires looking beyond the words themselves to the genetic comparison and the mathematics that supports the conclusion. This article aims to clarify what this phrase truly signifies so that readers can interpret their results with confidence.
Understanding Non-Exclusion in DNA Testing
The phrase “cannot be excluded” is a precise way for a laboratory to state that the tested individual’s genetic markers are fully consistent with being the biological parent. DNA relationship tests analyze specific locations on chromosomes called loci, which contain genetic markers known as Short Tandem Repeats (STRs). A child inherits one version of each marker, called an allele, from the mother and one from the father.
The laboratory compares the child’s paternal alleles against the alleged father’s alleles at approximately 20 to 24 different loci. If the tested man possesses all the necessary genetic markers that the child inherited from their father, he is “not excluded” as the source of those markers. This result means that the man’s genetic profile is entirely compatible with the biological relationship being tested.
The opposite conclusion, “is excluded,” is reached if the tested individual lacks the required alleles at multiple tested markers. This genetic data proves that the man could not have contributed the necessary DNA to the child, meaning he is not the biological father. Labs use the conditional language of “non-exclusion” because consistency alone only shows a match, not absolute proof across the entire human population.
Interpreting the Statistical Probability of Relationship
A non-exclusion result is always accompanied by a statistical measure that quantifies the strength of the genetic evidence. This measure is typically presented as the Probability of Paternity (POP) or Probability of Inclusion, expressed as a percentage. This percentage indicates the likelihood that the tested individual is the biological parent compared to a random, unrelated male from the general population.
A standard non-exclusion result will yield a POP of 99.9% or higher, which is considered conclusive in legal and medical contexts. The result is not reported as 100% because the calculation is an advanced statistical probability, not a complete comparison of the entire genome. The near-perfect percentage reflects that the tested man is many millions of times more likely to be the father than any other randomly selected man.
The underlying calculation for this probability begins with the Paternity Index (PI) for each individual genetic marker tested. The PI is a ratio that measures the statistical weight of the match at a single locus, factoring in how common that specific allele is within the population. All individual PIs are multiplied together to create the Combined Paternity Index (CPI), which represents the overall strength of the genetic match across all tested markers. A high CPI is then converted into the final Probability of Paternity percentage using a statistical formula.
Factors That Can Affect the Test Results
While a high Probability of Paternity generally provides near-certainty, specific biological circumstances can complicate the results and necessitate further scrutiny.
Closely Related Fathers
The most common complication involves cases where the potential father has a close male relative who could also be the biological parent, such as a brother or father. Closely related males share a significant amount of DNA, which means a standard test might yield a non-exclusion result for both men if only one is tested.
If the lab is not informed about the possibility of related fathers, the high probability of paternity could be falsely assigned to the tested man. The laboratory must be notified so they can perform an extended analysis using a larger number of genetic markers to find the subtle differences that distinguish the relatives. Testing both related individuals, if possible, is the most direct way to resolve this ambiguity.
Genetic Mutations
Another rare factor that can affect the interpretation of a result is a genetic mutation, known as a de novo mutation. A mutation occurs when a segment of DNA changes during the formation of the sperm or egg, meaning the child may possess a marker that the true father does not.
If the true father is tested, this mutation could cause a mismatch at a single locus, which might incorrectly suggest an exclusion. However, testing standards account for this by requiring mismatches at multiple loci to issue a full exclusion, ensuring that the rare occurrence of a single mutation does not lead to a false negative result.