Paternity Trio versus Mother Absent (Motherless) Paternity Test

This page shows an example of a DNA paternity test performed two ways:

  1. Mother Absent (motherless) (just the Alleged Father and one child)
  2. Trio (Mother, Alleged Father and one Child)

We show this example to illustrate that, in some cases, a mother absent test can produce an incorrect result if the alleged father is not excluded in the test.

The example below is generic and has been abbreviated to show only 10 markers, so it doesn't get too confusing to look through.  The intent is to show that a completely different result can be obtained with regard to exclusion or non-exclusion of an alleged father depending on whether or not the child's mother was also tested.


Why is it important to test the mother, alleged father and the child?

Test on the left: The alleged father shares markers with the child as seen in the motherless (mother absent) test on the left.   If you look at the highlighted markers, you can see that the alleged father possesses all of the obligate markers inferred in the test.  Therefore, in this particular case, the alleged father could not be excluded.

Test on the right:  Once the mother’s DNA marker information is included, one can see that the obligate markers that need to be contributed by the alleged father are not present in all marker systems.  Some of the markers attributed to the father in the mother absent case could only be contributed by the mother (right figure highlighted markers).  The mother's markers are actually identified in the test.  The markers that must be contributed by the father are listed as “Obligate Markers”.  It is clear that the father does not possess six of the obligate markers as indicated by the
X in the rightmost column.

It is now easy to see that the alleged father, who was not excluded in the motherless test, is CLEARLY excluded at 6 markers when the mother’s data is present.  If the alleged father was not excluded in the Trio test, he would possess the complementary markers at the 6 maker systems that excluded him in this example.

So how often would one see this occur in the real world?

This can occur in 1 in several hundreds to 1 in several thousands random, unrelated men of the same race, when a mother absent test is considered.  When the mother is also tested, the probability changes to 1 in tens of thousands to 1 in several millions of random, unrelated men of the same race.  These numbers can vary depending on the relative rarity of the markers present in the father and whether the rare markers were inherited by the child.  Lets put this into perspective. 

  • One in several hundreds to several thousands means that more than one man within a neighborhood or within a small town may be not excluded as a potential father. 

  • One in tens of thousands to several millions means that fewer than one man in a large city to less than 10 men in the USA would be not excluded as a potential father.

Obviously, the latter case is much more accurate.  Motherless (mother absent) tests can be done and the result is OK if the alleged father is excluded at two or more markers. The test is not as reliable if the result indicates that the alleged father is not excluded. The mother absent test should be reserved for extreme cases in which the mother is truly not available for testing.