With a paternity test, a question that often comes up is whether or not results can be incorrect if the two alleged fathers in the case are related. Here is a quick overview of relationship possibilities and how they may affect test results.
If alleged fathers are cousins
Whether the alleged fathers are first cousins or share a more removed cousin-connection, this type of relationship is distant enough that the DNA they do have in common will not cause any type of conflict with a paternity test. So in this type of scenario there is no need to be overly concerned if both fathers cannot be tested.
If alleged fathers are brothers
The possibility of this relationship causing problems with the results of a paternity test is higher, but not by a lot. Two people who are full brothers still have very unique DNA profiles.
It is extremely unlikely that two people who are only siblings would match the same child at all the markers tested for paternity. In a paternity test we generally test fifteen markers and in order to be considered the father, all fifteen markers must match. That being said, we highly recommend that both fathers be tested with the child. If that’s not possible, then you should definitely indicate on the paternity test paperwork that the other possible father is a brother and include the mother’s DNA as well.
If alleged fathers are twins
The DNA relationship between two alleged fathers who are fraternal twins is no different than the relationship between two brothers who are not twins. In this case, a paternity test should be able to determine which is the father. The DNA of identical twins, however, is exactly the same; so even if the mother is tested as well, paternity cannot be determined.