It is widely believed that DNA test will provide a simple “yes” or “no” result, however, the truth is that it is more complex than that, with a large majority of tests ending in a “inclusion” or “exclusion” result.
Interpreting A DNA Paternity Test
In most cases a DNA paternity test will provide a conclusive result. An alleged father will either be “excluded” from paternity or “not excluded”. The tests are based on the analysis of 15 genetic loci, each compared separately to obtain a paternity index (the 16th loci is the Amelogenin sex loci to confirm the sex of the donor). These indexes are then combined to produce a Combined Paternity Index (CPI).
What is the Combined Paternity Index?
A Combined Paternity Index (CPI) is derived from the Paternity Indices obtained for each genetic locus tested. The CPI represents the Probability of Paternity (POP) and is calculated on the basis of how common the genetic information found in your analysis is compared with the general population. The CPI can result in a 99.99%+ Probability of Paternity. Both the CPI and the Probability of Paternity should be quoted in a paternity test result.
What Does ‘Inclusion” Mean?
An inclusion is normally reported with a probability of paternity of 99% or more and a match is found at all genetic markers tested. An inclusion report will state that the alleged father "cannot be excluded" from being the biological father of the tested child. These three words often create a lot of confusion and unease, however, it must be noted that a result will never due to scientific reasons report an inclusion at 100%. One must always look at the combined paternity index (CPI) to help make results more understandable.
To make things clearer, it should be known that the absolute minimum to report an inclusion is a POP of 99% and a CPI of 100 (alleged father and child only) or 500 (alleged father, child and mother).
What Does “Exclusion” Mean
An exclusion is reported with a POP of 0.00%. In an exclusion result, it will be seen that with at least two genetic markers, there is a non-match. In an exclusion report, it is stated that the alleged father "was excluded" as being the biological father of the tested child.
When an exclusion is reported, a second, independent test is normally performed to confirm the initial result. Another possibility is that it could be an inclusion with a mutation.
Known mutations have a specific frequency in various racial populations and, often, that frequency is low. So, when the mutation frequency is figured into the formula for calculating the POP, it can possibly cause the POP to fall below 99%. To confirm mutations, it is always recommended that the mother is tested, if she has not already been included, or to perform extended testing of additional markers to help improve the statistical value of the result.