Paternity testing when two possible fathers are related

It’s not unusual for a customer to tell us there are two possible fathers for a child and that those two fathers are biologically related. Although two possible fathers who are related don’t share all their DNA (unless they’re identical twins), they do share enough that getting conclusive results for paternity testing may be problematic. So can paternity-test results under these conditions be trusted? Yes! Here are some relationship possibilities and how they may affect test results.

If Possible Fathers are Brothers

It’s important to remember that, although they are closely related, full brothers each have very DNA profiles that are still quite distinct from each other. The chances of two brothers who are not identical twins matching a child at each genetic marker for paternity testing are not likely. But since the relationship between possible fathers is so close, we still recommend that each of the men test with the child, if possible. At the very least, it’s essential to notify the lab that there are two possible fathers and that they are brothers. The lab can then test more genetic markers, if necessary. It’s also important to include the child’s mother’s DNA in testing. When you include the mother’s DNA, it helps analysts to more easily determine which data is coming from the possible father and which is coming from her.

If Possible Fathers are Twins

Fraternal Twins

If the men are fraternal twins, the DNA connection between them is the same as it would be for “regular” brothers. As when “regular” brothers test, it’s best when both men can test and the mother should also contribute a DNA sample to strengthen paternity testing results (see If Possible Fathers are Brothers, above).

Identical Twins

For identical twins, their DNA is as you would expect: exactly the same! With today’s level of technology, this makes being able to genetically differentiate between the twins for paternity testing purposes practically impossible and completely cost-prohibitive since it would require testing most of their genetic markers instead of the standard 16.

If Possible Fathers are Father/Son

A full 50% of the son’s DNA comes from his father, so if these two men are the possible fathers for a child, there is a high possibility of obtaining a “false positive” result if only one of the men participates in testing. Therefore, the ideal is for both men to test with the child. If this isn’t possible for whatever reason, then the lab must be notified ahead of time (so that additional analysis can be conducted) and the mother should definitely send in her DNA sample as well.

If Possible Fathers are Cousins

Even men who are first cousins don’t share enough genetic material in common to cause a “false positive” for a paternity test: the connection is just too far removed to make a significant difference.

One Final Thought . . .

To repeat the most important points: In all cases where two possible fathers are closely related, it’s best if both men can test at the same time.  If there is a close genetic connection between possible fathers, and only one man can or is willing to test, it’s important to notify the lab about the biological relationship when submitting DNA samples: better safe than sorry!

Don’t hesitate to call us at 800-929-0847: We’re here to help.


  1. I have a son who is almost 30 there is a possibility between 2 men, not related one is deceased the other is alive & well. Which is the best way to get a DNA test on the man that’s still here. Only thing is he lives in a different state?? Help please? Can it be done by mail?

    • Hi, Ruth Ann. The possible father living in a different state is no problem. We would just send testing materials to you and your son at your address and then send a kit to the man at his address. Just give us a call at 800-681-7162 during business hours.

  2. My daughter had a dna test done and the results came back 99.96 but the other alleged father who is the other mans brother was not tested nor mentioned to the testing agency is there any possible way that the results could be wrong and his brother is in fact the child father?

    • Hi, Sami. In cases like this, a false positive may occur. It really depends on how much DNA the brothers happen to have in common at the locations tested. It’s essential that your daughter be tested again, hopefully this time with both men. If that’s not possible, then the lab definitely needs to be made aware of the other alleged father and his biological relationship to the man being tested BEFORE they do the testing.

  3. Two men get tested to determine the paternity of their daughter (they both think they are the father).
    After the tests, one of them is the father. Neither one of the men suspects they are related to each other. Let’s presume they are.
    Would the DNA test done to determine paternity inadvertently discover the relationship?

    • Hi, Julia. That’s an excellent question! It depends on how much DNA the two men share in common, whether they’re brothers or father/son, etc. What I’ve definitely seen a few times is this: a man is excluded as the father (meaning he’s not considered the biological father), but the lab made a notation in the report of the possibility that the actual biological father is related to the man who was tested.

  4. Hi, Both fathers in question are dead but my sisters father is the brother of the man I think is my dad. He is also dead.. Can I do a DNA TEST with my sister to see if our fathers are related.. We both have the same Mother.

    • Hi, Dani. Because you and your sister share the same mother, you already know you are half siblings and a test is not necessary to determine that. You may want to test with someone whom you are certain is related to your possible father in the first degree: like a known child of his. I suggest you give our experts a call so you can walk through your situation with them on a one-on-one basis and see what your options are: 800-681-7162.

  5. Hi, what if the two potential fathers are half brothers ( they share the same mother but different dads), is it possible to accurately determine the father of the child if you only have samples from one of the half brothers?

    • Hi, John. The answer is yes. However, two things are very important:
      (1) The mother of the child should be tested also
      (2) The lab must be notified ahead of time that another alleged father is the half-brother of the man being tested. This way, they can take this fact into account when doing their analysis.
      Hope this helps!

  6. I am interested in clearing up the relationship between myself and sibling. I have a suspicion that my father and my siblings father were brothers (same mother) We are older and parents all deceased. Can this be definitively shown?
    Thank You

      • Like Bob (Nov. 15 2018 above) my brother and I share the same mother. She is still living. The three of us have been DNA tested: mom and I via 23andme, and my brother by MyHeritage. We’re confused in that my brother and I only share (as per MyHeritage) 38.4% DNA. I thought we would share closer to 50%. Since our shared DNA percentage is much lower, could this mean we might have separate fathers, who were perhaps brothers? Or is this 38.4% in the realm of same parents?

        • Hi, Steve. I talked to our Chief Science Officer personally about your question. He thinks that 38.4% if a little low, but it’s not out of the realm of possibility for two full brothers. It really just depends on what you inherited from both parents in the genetic lottery. It is possible that you had fathers who were brothers…have you asked your mother about this? If it is possible that the fathers were brothers, then doing a Y-STR test with you and your brother wouldn’t do any good, since the two fathers would also share the same Y chromosome. You could do a sibling vs. half-sibling DNA test (with your mother including her DNA) while notifying the lab ahead of time that there may be another alleged father who is the brother of your presumed father.

          • Thank you! Great information and so quickly. Will connect with my brother and maybe mother as well, however this could be a touchy topic. She would probably be up for another DNA test and I could just leave it at that. My brother and I are quite interested in finding out more. Again – thank you!

          • Yes it might be a touchy topic, for sure. You’re welcome, and good luck. If you have any other questions, feel free to reach out again.

  7. Hi, I have a couple of questions I thought hopefully you might be able to answer…1st if a DNA test came back only 97% probability, would that mean I’m the Father? And second my kids Mom & my mom are sisters, my mom had here tubes tied before I would have been born. But just by some miraculous chance my mom did conceive me after that, which would obviously make my kids mother myself maternal cousins, what kind of result would you get?

  8. I have a question which I have gotten some conflicting answers on. The question I have is if my fathers brother (my uncle) is the other possible father of my child, is that a close enough relative to me to produce a false positive for me on the paternity test ? I already tested and got back the result of 99.99% with a CPI of 19,091. I am wanting to know if we are close enough relatives for it to affect the results of my test ? I would really appreciate your help on sorting this out. Thank you.

  9. I am not sure if the father or son is my child’s father. Will the paternity test be clear if only the father is willing to be tested and the son is actually the father?

    • Hi, Anna. Excellent question! This is very important: If only one of them can or is willing to test, you MUST let the lab know ahead of time that there is another possible father and what their biological relationship is. That way, the lab can take this into account in their analysis and test more genetic markers, if necessary, to obtain conclusive results. It’s also helpful for you to contribute your DNA to the test as well.

      • What will the DNA results show if I decide to just wait and get results with father being tested for paternity as the son has refused? Will the test results be negative or inconclusive if he is not the father?

        I have a suspicion that the son is actually my child’s father but I can proceed further if the results are not conclusive.

        • Ann, with today’s technology and methods, an accredited lab should never return an inconclusive result for a paternity test. When the father tests, it’s absolutely essential that you let the lab now ahead of time that the other alleged father is the son of the man being tested. That way, the scientists can take that info into account when doing their analysis. They can test additional markers, for example.

  10. Hello! I been having this gut feeling ever since my ex took the DNA test about 3 years ago that he is the father, but when he did the test, the results came negative, and he was very upset, there is another guy who I am having a problem contacting him to also get the paternity DNA test, I even got him into child support JUST for him to do the Test! what are the chances if my ex takes a second DNA blood test but this time would be him, my son(9) and me? I mean how different the tests are compared to the swab, and only taken by the alleged father and child? And also having the same birth marks can prove the alleged father is the one? (meaning my ex) even though he already took the test and it came negative?

    • Hi, Juliet. DNA collected via blood and via swab is exactly the same…one method is not more “accurate” than the other, in terms of results. If DNA for the same exact people who did the first test is submitted again for the second test, you can expect to see the exact same results, especially if you used an accredited and reputable lab like ours. A birth mark alone is not proof of biological relationship. All this being said, if you still have doubts, why not do another paternity test with your ex? I recommend doing a legal, witnessed test so that results can be used for court, if necessary.

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