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 Interpreting Results
The DNA testing process is the most accurate testing possible. When the DNA paternity test includes a mother, child, and an alleged father, all of the child’s alleles (alternate form of a gene) inherited from the father can be determined by subtracting the known alleles inherited from the child’s biological mother. If the alleged father contains all the necessary alleles, the man is not excluded as the biological father. If the alleged father fails to share the required allele at three or more locations, he is excluded as the biological father. When only the child and alleged father are tested, failure to share common alleles at three or more locations indicates an exclusion.
The Paternity Index (PI) is the genetic odds in favor of the alleged father being the biological father of the child. Each DNA location (locus) has a specific PI that is calculated. This calculation represents the chance of the alleged father contributing the necessary allele compared to a random, untested man in the population. The Combined Paternity Index (CPI) is calculated by multiplying the individual Paternity Indices. As an example, if the CPI is 100,000, this means that the alleged father is 100,000 times more likely to be the biological father of the child than a random, untested man in the population. If the alleged father is excluded at three or more locations, this will result in a CPI of zero (0), indicating the alleged father is excluded as the biological father of the child. The Probability of Paternity is expressed as a percentage on your test report. This percentage indicates the chance of paternity based on the complete DNA test results.
Our laboratory often achieves a Probability of Paternity of at least 99.999%—indicating that there is only a minute, 0.001% probability that another random individual in the population could have the same paternity test results and be the child’s biological father.
Occasionally, an alleged father will not have the necessary allele at one or two locations. This could be the result of a mutation in the DNA. When this occurs, our laboratory conducts additional analysis to determine whether the alleged father is excluded or not excluded. If no other locations indicate a non-match, a Paternity Index (PI) reflecting the mutation rate is listed on the report. Alternatively, these non-matches may also indicate that a close biological relative of the alleged father (possibly a brother or father) could be the biological father of the child. Additional testing can be performed if this is a possibility.
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