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.
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!
My dna test came back as 97.99% with the man that was tested. He has a brother who might be my father as well. That man was not tested. Does that 97.99% mean there is a change his brother is my father and not him?? Since it was not at 99.99% result
Hi, Samantha. Was the lab notified ahead of time that the other possible father is the brother of the man tested? If not, then new testing is needed.
Hi, is there a test to determine if I am the 1st cousin with a Whopping 1479Cm match, or am I a half sister to a man who is possibly my uncles child. This man is so close to to me , our DNA is like 3/4 sibling.
Hi, Cathy. Please contact us directly at 800-681-7162 (M-F, 8 AM to 8 PM Eastern) to speak with one of our experts and determine testing options.
Hi, I was wondering if you guys could do a paternity test on a baby who’s still in the womb to determine who’s the father between 2 brothers? The baby is about 4 months along.
Hi, John. I originally gave incorrect information, sorry. We cannot do prenatal testing if both possible fathers are close relations, even if they test separately.
Hello DDC,
I am curious about this answer. On your page you state you can not do a non-invasive paternity test (DNA test while baby is in the womb) if you are testing close relatives (two full brothers or father/son). But now you state in response to John that IT MIGHT BE POSSIBLE IF BOTH BROTHERS PARTICIPATE. Help!! Please further explain and clarify.
Hi, Nan. You are correct. I didn’t answer John’s question correctly the first time and I apologize for the confusion. I have since edited my response to him.
I was tested for paternity but was never shown results. I suspect a close relative to be the father. But The mother says the results were that I am the father. Could she have been given a 50% or 75% percentage because the father is my relative. Or would It just state Postive or negative for paternity?
Hi, Thomas. It would just state whether it’s an inclusion or an exclusion for paternity.
Hi, i have results from paternity test between father,mother,son and daughter. It’s possible that both baby’s have same alleles at 9 loci from 24 tested? Can by my half brother (same father) father ?
Hi, Matthew. Without seeing all the data, I cannot confidently provide an answer. Sorry! I suggest you contact the lab where you tested since they have access to all the data and ask questions.
I can send report with loci and alleles to you. It’s possibility that the lab maybe manipulated something. Can you recognize false positive results?
You are welcome to contact us privately on the DDC Facebook page and we’ll see if we can help: https://www.facebook.com/DDCPaternity
I have adopted two girls, both of which have the same birth mother. We have been told who the father is of one, or at least who she believes is the father but there were multiple possibilities of the other. Is there a way to just run the DNA profiles of my two girls to see if they have the same father? Or would we have to have the men involved?
Hi, Julie. You can do a full-sibling vs. half-sibling test. Ideally, the birth mother would contribute her DNA as well to optimize conclusiveness of results, but since yours is an adoption case, that may not be possible. Contact us for a free consultation at 800-681-7162 (M-F, 8 AM to 8 PM Eastern).
Hello, is the test reliable if used sample from grandmother (father’s mother) , and her grandson ?
Hi, Adam. Yes, it is, especially when also including the child’s mother’s DNA in testing. You can learn more here: https://tests.dnacenter.com/relationship-testing/grandparent/
Will I get the correct results if both possible Fathers are related to the mother ( possible fathers are a father and a son and the mother is the daughter to the same father and mother as the son)
Hi, Puleng. All those complicated relationships will need to be explained to the lab prior to testing, and then a decision can be made whether or not testing is possible.
I tested potential father number one ( brothers ) and told them and got a negative then yrs later had to test potential father number 2 and got a 99.97 but says it found a mutation and doesn’t exclude close relatives ie brother or father .. is it possible that potential father number one could b . Mother was not tested either time .
Hi, Elizabeth. If potential father #1 was excluded in a separate test, then that is conclusive for him (assuming he submitted his own DNA for testing). Potential father #2 is considered the biological father with a 99.97% probability of paternity, and since you already know it cannot be potential father #1 (assuming he submitted his own DNA for testing), then you can conclude the result for father #2 is correct. All that being said, unless you know for certain that father #1 submitted his own DNA for testing, you may want to test him again. This time include the mother’s DNA and inform the lab that the man being tested is the brother of the other man so that this information can be included in their analysis.
I am trying to find out who my dad is. It’s between a uncle and nephew but the uncle and sister (mother of the nephew). Both sister and nephew are both deceased. I am wondering if I do a DNA on the uncle would I be able to determine if he is or isn’t my father?
Hi, Crystal. Chances of obtaining conclusive results are much higher if your mother is also willing to participate and it is absolutely essential that you let the lab know of the other possible father, even if he is deceased. I suggest you contact us directly for a free confidential consultation at 800-929-0847 (M-F, 8 AM to 8 PM Eastern).
My brother and I got tested to try and find out if my dad was actually my uncle or not, the results came back 99.78%
Is that 0.12% anything I should worry about?
Hi, G. Did you do an avuncular test or a paternity test? Your statement isn’t clear. I suggest you contact the lab where you tested to have your questions answered.