DNA Testing Services
Exclusion in DNA Tests
DNA paternity tests are used to prove whether a suspected father is either the biological father or if he is excluded as a candidate for being the father. The term used for this is called exclusion and is determined by analyzing the markers found in the child's DNA as compared back against the father's DNA. In this article we'll look closer at exclusion and describe the process used to determine it.
When DNA tests for paternity are performed they are done by taking DNA samples from the child, biological mother and suspected father. Those samples are then used to help identify and classify the 46 chromosomes found in the child's DNA. Half of those chromosomes will come from the mother, the other half will come from the father. Once the mother's chromosomes have been identified and isolated the job of DNA analysis is to compare the remaining 23 chromosomes against possible fathers.
All it takes is one mismatch to prove that the suspected father is indeed not the father. However, as with any test, the more mismatches the greater the probability of exclusion is raised. This can be a tricky subject, let's say for instance that 22 out of 23 matches are made. This means that the suspected father is not the father but someone closely related to him probably is – a brother perhaps or maybe his own father. It could also mean that the test was not analyzed correctly. If such a situation were to play out the first action would be to re-analyze all the DNA samples and perform new matching using different probes to bring out distinctive markers.
If all 23 chromosomes match up then the suspected father is indeed the biological father. Thanks to the uniqueness of DNA it is a highly accurate testing procedure and cannot be fooled. For this reason DNA paternity testing is quickly replacing blood testing for paternity in many locations.
If for some reason the mother's DNA is not available, it is still possible to determine exclusion but the probably falls to 99.6% instead of the standard 99.8% -- it is important to remember here that courts require a 99.5% probability rate or higher for a test to be accepted as proof. Without the mother's DNA there are a total of 10 independent tests that are performed on the suspected father's and child's DNA. Again, if the chromosomes do not match up in any position then the father is excluded with a probability of 99.6%.
False exclusion can take place when DNA samples are contaminated or not enough independent tests are run against the DNA. Standards require that a minimum of eight tests be performed independent of each other in attempts to match the makers for the DNA. This means that eight different probes are run against the DNA to extract markers that can be matched up.
DNA mutations are rare, but can occur. If such a situation occurs then only one marker may not match (as explained above) or there may be a match on some of the tests and not others. If such a situation occurs extended testing of the DNA may be performed to resolve the conflicts. Though costly, this is the only accurate way to prove inclusion or exclusion for parenthood.