A DNA relationship test, most commonly a paternity test, relies on comparing genetic markers between individuals to establish a biological link. This analysis works by examining specific regions of the DNA, known as short tandem repeats (STRs). Since half of a child’s genetic material comes from the biological father, scientists look for shared markers to confirm this inheritance pattern. Because of the complex nature of population genetics and statistical analysis, a test result cannot be a simple “yes” or “no.” A standardized probability threshold is required to provide a definitive answer.
The Accepted Threshold for Inclusion
Accredited laboratories must meet a specific statistical standard before declaring a biological relationship to be confirmed, or “included.” The industry standard for a positive paternity test result is a Probability of Paternity (POP) of 99.0% or higher. Most modern DNA tests achieve a probability of 99.9% to 99.99% or even higher.
This high percentage represents the likelihood that the tested man is the biological father compared to a random, unrelated man. The result is never reported as a perfect 100% because it is impossible to test every male on the planet to definitively exclude them. The conclusion is typically worded as the alleged father being “not excluded” from being the biological father, reflecting this statistical certainty.
Understanding Probability of Paternity and Indices
The percentage reported in a DNA test result is known as the Probability of Paternity (POP), derived from a more fundamental calculation. The process begins with the Paternity Index (PI), a value calculated for each individual genetic marker, or locus, tested. The PI expresses the strength of the genetic match, comparing the likelihood that the tested man passed on the child’s inherited allele versus a random man.
To determine the overall statistical weight of the evidence, the individual Paternity Index values from every marker are multiplied together to create the Combined Paternity Index (CPI). If a laboratory tests 20 markers, the CPI is the product of all 20 individual PI values. The CPI represents how many times more likely it is that the tested man is the father compared to an unrelated man.
The Combined Paternity Index is then converted into the final percentage, the Probability of Paternity (POP), using a mathematical formula that incorporates a prior probability. Laboratories typically use a prior probability of 0.5, or 50%, which assumes an equal chance that the man is or is not the father before the DNA evidence is analyzed. A CPI value greater than 1000 is considered strong support for paternity and translates to a POP greater than 99.9%.
Distinguishing Inclusion from Exclusion
DNA testing yields two definitive outcomes: inclusion or exclusion. An inclusion result means the tested man “cannot be excluded” as the biological father, supported by the high Probability of Paternity (POP). This statistical conclusion is reached only when the child’s inherited genetic markers align perfectly with the alleged father’s profile at every locus tested.
Conversely, an exclusion result is definitive and absolute, stating that the alleged father is ruled out as the biological parent. This finding requires mismatches at a minimum of two or more different genetic markers. A single mismatch is often treated with caution and may be investigated further, but multiple mismatches confirm a break in the biological inheritance pattern.
When a man is excluded, the Probability of Paternity is reported as a conclusive 0%. Exclusion is considered 100% certain because the fundamental law of genetic inheritance has been violated. The inclusion result, while extremely high, is a statistical likelihood, whereas the exclusion result is a direct biological contradiction.
Factors That Influence Result Certainty
While the Probability of Paternity often reaches high values like 99.999%, several factors can influence the final certainty reported. The inclusion of the mother’s DNA sample significantly increases the certainty of the calculation. The mother’s sample allows analysts to subtract the markers the child inherited from her, isolating the paternal contribution and strengthening the statistical comparison.
Genetic mutations are a rare but possible factor that can complicate the analysis. A mutation is a spontaneous change in the child’s DNA that can cause a single mismatch at one locus, even if the tested man is the biological father. The mother’s sample is particularly helpful in confirming that the single mismatch is a mutation rather than an exclusion.
The most significant factor that can lower the final percentage is the possibility that the other potential father is a close biological relative of the tested man. Since close relatives, such as brothers or fathers and sons, share a greater percentage of their DNA, the statistical calculation becomes more complex. Laboratories may need to analyze a higher number of genetic markers to achieve the standard level of certainty in these cases.