The ABC show “Brothers and Sisters” once aired an episode in which the two-year-old daughter of Tommy Walker needed a liver transplant. In an earlier episode, when it was discovered Tommy was not able to father children, he asked his two brothers to become sperm donors so that he and his wife could have a child. Since the liver transplant needed to come from the actual father, it was time to find out which of the two brothers was the biological father.
This brings up two often-asked questions:
How difficult is it to determine the paternity of a child when two fathers are closely-related (brothers)? Does testing two brothers decrease the accuracy or probability of the test?
The truth is DNA testing is very accurate and, unless the brothers are identical twins, the test will accurately reveal which one is the father. The biological father’s profile must match the child at ALL locations (loci) tested: usually 15 markers. It’s extremely unlikely that a brother would match a niece at all loci, unless he’s an identical twin to the brother. In that case their genetic profiles will be exactly the same and you wouldn’t be able to tell which one was the father.
In cases like this, we recommend testing both brothers (both alleged fathers) and including the biological mother in the test. By including the mother’s DNA, we can identify which markers in the child’s DNA came from mom and focus on the DNA that must have come from dad. If both brothers are tested with the biological mother, one will emerge as the biological father without question.
Any two people tested, even if unrelated, will likely share at least some markers in common, but a biological father and child will share all markers tested. Including the mother, testing additional markers or loci if needed and testing the other brother will identify more mismatches between the uncle and child and show stronger matches between father and child.
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