Testing “Brothers and Sisters” for Paternity
Over the weekend, the ABC show Brothers and Sisters 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 donate so that Tommy and his wife could have a child. With the liver transplant needing to come from the actual father, it was time to find out which of the two brothers was the biological father.
This brings up some questions that I am often asked.
- How difficult is it to determine between two brothers, the paternity of a single child?
- 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, then their genetic profiles will be exactly the same as his brother and you cannot tell which one is the father).
In cases like this, we recommend testing both brothers (both alleged fathers) and including the biological mother. By including the mothers 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 (1, 2, 10), but a biological father and child will share ALL markers tested. If only one brother is tested and the mother is NOT tested, there is a possibility (extremely small) that he could match the child at all locations. This rare case could occur because he is genetically related as an uncle and because he randomly matches the child as any two people might. This would make him appear as though he were the biological father. Including the mother, testing additional markers/loci, and testing the other brother will identify more mismatches between the brother/uncle and child and show stronger matches between brother/father and child.
Hi Natasha,
Thank you for your question. This is a very complicated situation, however, it is something that can be looked into. When you submit samples to the lab they do need to be made aware of the relationships. There may also be others that the lab would want to include in the testing as well. I would be happy to speak with you more about this. If you have other questions please contact me directly at ashaw@identigene.com
can i do an dna test with the dads sister ?
Hello Katisleidys,
It’s always best to complete a paternity test with the alleged father and child, however, if the alleged father is not available you can choose the aunt/uncle test if the sister of the alleged father is a full sibling. For this test you would want the mother of the child to participate as well if at all possible.