A paternity test result is highly accurate and due to the specific loci (locations on the DNA) that are examined and how we inherit these loci. DNA paternity testing therefore is a very powerful form of testing. In a test including samples from the mother, child and alleged father, the probability of paternity is 99.99% or greater when an alleged father’s DNA profile matches that of the child for all the genetic markers. On the other hand, an alleged father is 100% excluded from paternity if there is a mismatch for three or more genetic markers between the profiles of the child and alleged father.
When the paternity test only involves a child and alleged father (i.e. a motherless test), the information provided by the mother’s DNA profile is unavailable. Nonetheless, when there is a perfect match between the DNA profiles of the child and alleged father, the probability of paternity is generally in excess of 99.9%. A mismatch in the two DNA profiles for three or more genetic markers implies with 100% certainty that the alleged father is not the child’s biological father.
The accuracy of the test result increases with the number of genetic markers included in the DNA profile (the number of markers tested depends on the laboratory). Thus, the result of a test that uses a 16-marker profile is likely to be more conclusive than one using a 6- or 13-marker profile. It is therefore important to select a paternity test that uses an adequate number of markers, especially when only the child and alleged father are tested.
Since family members are more likely to have similar genetic profiles, when two alleged fathers are related the probability of paternity may be lower than in standard paternity tests. In such cases it is always best to test both alleged fathers, since the one who is not the biological father can be excluded with certainty. If the two alleged fathers are identical twins it is not possible to identify the biological father, since identical twins have identical DNA profiles.
Why doesn’t a Paternity Test give 100% as a Positive Result ?
The reason that it is not possible to determine biological paternity with 100% certainty is that there is always a very small possibility that the profile of the alleged father matches that of the child purely by chance. The likelihood of this happening is generally well below 0.001% (or 1 in 100000) and it depends to a large extent on the ethnic origin of the individuals involved. The certainty of biological paternity generally increases with the number of genetic markers analysed.
Example 1
Example 1 :The figure shows the result for one microsatellite marker from a paternity test that includes samples from the mother (top row), the child (middle row), and the alleged father (bottom row). In this example, the maternal marker that has been passed to the child is number 6. This means that the other marker present for the child (number 7) must have been inherited from the father. In this case the alleged father matches the child, since one of his markers is indeed number 7. This procedure is repeated for all microsatellite markers used in the test.
Example 2
Example 1 : The figure shows the result for one microsatellite marker from a paternity test that includes samples from the mother (top row), the child (middle row), and the alleged father (bottom row). In this case the maternal markers are 29 and 30. This implies that the child has inherited 29 from the mother and the 31.2 complement must have been inherited from the biological father. Since the alleged father does not possess this marker it is unlikely that he is the biological father. In practice this mismatch between the child and alleged father’s DNA must be present in at least three markers to exclude paternity with certainty.
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