Case Studies

For example, consider the following case:
Locus Biological Mother
(not tested) Alleged Father Child Parentage Index
D2S1338 -- 12, 13 10, 12 1.845
D2S1358 -- 8, 11 11, 14 2.714
D8S1179 -- 21.2, 32 19, 21.2 2.675
D19S433 -- 15, 18 12, 15 7.338
In this case, the probability of paternity is 98.2896% (the product of all the parentage indexes). The result is inconclusive (because it must be greater than 99% or 0%), yet the alleged father and child match at all locations. Now, add the biological mother's sample to the DNA paternity test:
Locus Biological Mother
(Mother A) Alleged Father Child Parentage Index
D2S1338 8, 10 12, 13 10, 12 3.489
D2S1358 14, 17 8, 11 11, 14 5.114
D8S1179 15, 19 21.2, 32 19, 21.2 3.619
D19S433 8, 12 15, 18 12, 15 15.309
The probability of paternity increases to 99.9541%. Why? In the first example, one of the two markers from the child and alleged father match at each location. However, we don't know which of the child's markers comes from his mother and which must come from his father. By testing the child's mother, we see which of the child's markers must have come from the father. In the second table, the paternity index is increased.
Not only does the child match the alleged father, but the match is with the marker that must have come from the child's true biological father (since we can see which marker came from the child's mother). In fact, the index value is higher at each location because the biological mother participated in the DNA test.
But, what if the mother's DNA produced different markers?
Locus Biological Mother
(Mother B) Alleged Father Child Parentage Index
D2S1338 8, 12 12, 13 10, 12 0.000
D2S1358 14, 17 8, 11 11, 14 5.389
D8S1179 21, 21.2 21.2, 32 19, 21.2 0.000
D19S433 12, 15 15, 18 12, 15 0.786
With this data, the probability of paternity becomes 0%. The alleged father is not the child's biological father. The biological mother must match the chlid at all locations. We can see that this alleged father does not truly match the child's markers that must have come from the child's true biological father. In some places where he appeared to match the child's markers, the markers clearly come from the biological mother.
Note that there are still some matches between the alleged father and child. If this alleged father is truly the biological father, he must match at all locations (almost any two people will have at least some matches, but a father-child relationship will show matches at all locations). Even a few mismatches can be enough to exclude the alleged father from being the child's biological father. In this case, DNA testing the biological mother turns an inconclusive result to a definite "no" -