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.
If the suspected biological father is deceased, would it be possible to test his brother to determine if the child shared DNA with that family?
Sure, you can do a DNA test between an uncle and a child. We call this an Avuncular DNA Relationship Test. This will show the probability of a relationship between the father’s brother and the child. This test is $495.00 and can take two to four weeks to complete. If you would like more detailed information on this type of testing you can visit our website at: http://www.dnatesting.com/dna-test-relationship/avuncular-dna-testing.php or contact our client support team at 888-404-4363.
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If testing is done and it is found a man is not the father, and then it is discovered his brother was sleeping with the same woman, what is the probability of the child being his since the test came back negative on his brother?
Parker, I apologize, but there is no fixed probablity to give you in this situation. Even though one brother was excluded as the father that does not automatically exclude the other. Both would need to be tested to prove that they are in fact not the father of this child. I hope that this information will be helpful.
Is it possible for an avuncular test for two aunts to come back in the really low probabilities, i.e. 2% and 14% probabilities, if the biological father is indeed my father??? We assume the siblings all share the same parents. We have not done a DNA test with the father’s DNA, and I was astounded with these low results from testing the two sisters of my father. Is this enough to make a paternity conclusion that my father isn’t really my father?
You said “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”.
What happens in this case: two brothers (not twins) had sex with the same girl. Girl tells brother A that he is the father but brother B is the actual father. Brother B is unreachable and brother A wants to make sure he is the father so he gets a paternity test. Both brother A and the girl are available for the testing. Does he need to inform the lab technician about this situation? And how accurate could the results be? What kind of result would he need to get to be sure that he is not the father?
what does one do when two brothers are alleged fathers, but only one was named as the father, tested 99.9. Should I be satisfied that he indeed is the father or maybe his brother is the father.
Here’s the story…My husbands mom and aunt were both married to the same man. That man is possibly my husband’s father but is not the man who raised him. This man is now dead and my husband is wanting to know if his cousins are actually his cousins/half brothers and sisters by the same father. Is there a way to know since they would have genetics in common through their moms being sisters and the father now being dead.
I apologize that I did not get back to these comments sooner. Mandy it is difficult to speculate whether Avuncular testing might return low results like yours when their sibling is in fact your father. Because siblings only share a certain amount of DNA Avuncular testing can sometimes give results that one might not expect.
Matt: If the biological mother is tested it does help with the conclusive nature of the testing. However, we would still appreciate being informed of two alleged fathers being brothers to ensure all proper protocols are followed to provide accurate and conclusive results.
Dee: With results that high I would say that the father tested is the biological father. However, if you still have concerns I would suggest having a DNA test done on the other brother.
Charlie: It can be difficult to tell cousins and half siblings apart. This is because there is already an established biological relationship. If there is a male half sibling the best test I could recommend would be a Y-Chromosome test as this will only test if the two share a common male ancestor. You can find these tests on our website.
So my dad was with this lady and she got pregnate but my dad thinks its his brothers. So my dad got a paternity test just with him and the child and it came up as 99% could it be his brothers?
So, If two brothers were suspected to have possibly been a childs father and one brother had test done and results were 91.7%, would you say that he was the childs father?
My hubby was tested and the results came back positive. I am not sure if they tested her mother, but we are suspecting that his brother messed around w/ her as well. I am paying for another dna test for my husband and her but if this comes back negative would they be able to tell if it were a family members dna? If they were close to matching?
hi i have two daughters and would like to prove they dnt have the same father, at the moment i cant get the fathers to do a paternity test and im unsure which test i should now do to prove this. could u advise me of which test i should get? thankyou
Philomena: It’s difficult to say. The best choice, when looking for definitive results, is to test EVERYONE. Including the mother would strengthen the result. Including the other brother as well would help even more.
Nikki: That’s a good question. If the test comes back that your husband is excluded as the father, the only thing to know for certain is that he is not the father. If your husband and the child match at several locations, it’s possible that your husband shares those markers with the child’s father. However, it could be because he and the child’s father are related, or it could be that those markers are just very common.
Becky: The short answer is that you need a half vs. full siblingship test. This would test both your daughters, and yourself, and would give you the probability that your daughters share the same father. This test however isn’t as conclusive as a paternity test, and would give you only the LIKELYHOOD that they are full siblings. Siblingship Tests CAN be inconclusive. A paternity test would be less expensive, and would definitively exclude one man from being the father of both girls (with 99.99% certainty).
My grandmother slept with two men who were brothers. My mom was born shortly after. Then my uncle & two aunts were born. They were all raised believing brother A was their father, but once he passed, the family started saying that brother B was my mom’s real father. Problem is, both brother A & B are dead. Brother B has a son as well, so is there a way for my mom to compare her dna with that of my uncle, aunts & her “cousin” (who actually might be her half-brother)?
Hi Natalie, Your mom could do a DNA siblingship test to determine if her brother is her half-brother or full brother (this would be a “Full vs Half Siblingship” test.) When doing DNA relationship testing, it’s best to use the closest (biologically) relative, and in this case, that would be one of her siblings.
When doing these kinds of tests, it is especially helpful to include the biological mother (your grandmother). If you have specific questions you can call one of our test consultants at 888-404-GENE.
Can you tell me if I can have my sister and I tested to see if we have the same father without testing anyone else? I am 42 and I was 41 when my mother admited that my dad might not be my dad. I do not want to hurt the man that I have grown to know and love to be my father. Please advise.
I dated this guy (brother A) for three years. We broke up and occasionally had sex. I stopped having sex with him and about a month-two months later I went out to the bars with his brother (brother B) and we ended up having sex a couple of times when we were drunk. I then found out I was pregnant. We had a paternity test done with me, my daughter, and brother B. The test results were 99.89 %. We never had brother A tested and now I am starting to wonder if I made a terrible mistake not having him tested. Is it possible that brother A could be the father?
http://www.dnatesting.com/more-dna-tests/sibling.php
Neva, you certainly could! What you would want to do is a Full vs. Half siblingship test. When doing this test, it’s best if you can include your mother. This will strengthen your result.
The test can still be done with just you and your sister, how ever it is possible that without your mom, you could have an inconclusive result.
Because you and your sister (or half sister) both received DNA from the same mother, including your mom in the test will allow us to isolate the DNA that comes from her to be sure we are comparing the DNA that you and your sister received from your father.
could you please tell me if its possible to do a dna test on a man and a woman who have different mothers but potentially the same fathers?
Hi Anne,
You sure can do this test. It would be a sibling ship test, Half vs. Unrelated. This test would tell you the likleyhood that you share a parent in common (are half siblings) vs having no common parents (un-related).
http://www.dnatesting.com/more-dna-tests/sibling.php
I have just had a DNA test with someone I think is my uncle, however the test has come back “inconclusive”. The lab has offered an extra test but I am unsure if this is going to make any difference. Can an inconclusive test have any chance of coming back positive?
Dear Karen,
An inconclusive result is just that: inconclusive; meaning that the lab cannot tell whether the person is the biological father. There is no way to know for sure without additional testing; inconclusive tests do not “lean” one way or the other. However, there is still a way to answer the paternity question.
One common issue with inconclusive results is that the tested alleged father IS NOT the biological father, but IS RELATED to the biological father (and also related to the tested child, though not the father of the child). An uncle will typically share more DNA in common with a niece or nephew than someone else who is unrelated to the child. In most cases it is clear that the tested father is not the father but still related to the individual. However, in some cases the lab needs more information. The best option is usually to include the biological mother in the test.
Remember, paternity tests work by looking for matches between your DNA and the tested father’s DNA. At every location the lab reviews, you (the child) have two allele numbers. One of these numbers comes from your biological father and the other from your biological mother. The biological father will match at all locations. Without the mother, there are TWO chances for the tested father to match the child at each location, because he can match EITHER of the child’s allele numbers. If the mother is included, the lab now knows which of the child’s alleles belong to the mother and which belong to the father. Now, the tested father has only ONE chance to match the child at each location. When the mother is included, the strength of any match increases because there’s a higher chance that the match shows paternity – the lab knows for sure that the match is consistent with the father because they eliminated the DNA that came from the mother. Also, there may be some matches that now become excluded (because the tested father matched an allele number that came from the child’s mother and not the child’s father).
The strength of the matches determines the probability of paternity. If the probability is less than 99%, the test is considered inconclusive. Including the mother will make the result clearly fall one way or the other (but it’s impossible to know which way it will go until she is tested). Also, testing the other possible father usually helps, too. Often an uncle may show an inconclusive result but the true biological father will show a very conclusive result. In either case, though, including the mother helps strengthen the test results.
For a detailed discussion about DNA paternity testing process and including the mother, please see these links:
http://www.dnatesting.com/genetipedia/article/paternity-test-process
http://www.dnatesting.com/genetipedia/article/why-test-the-mother
Dear Kate, Would a DNA test determine if my brothers have the same biological father that I have even though our father or fathers are deceased?
Hi Robert,
Yes, a DNA test could determine if your brothers are full siblings (share the same parents) or are half-siblings (share one parent) even without the father’s DNA.
For the strongest result, the mother’s samples should be included, if at all possible.
For more information on Siblingship testing see: http://www.dnatesting.com/more-dna-tests/sibling.php
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Hi Kate!
Imagine that a mother has in a locci (Penta D) the alleles 9-10 and the child has in the same locci (Penta D) 9-10. In this case how do you know which allele come from the father? Could be 9 or 10? But according to your coments theres only one biological fahter that would match in ALL loccis and then you will know wich allele come from mom in this locci? Correct?
I apreciate you would aswer this?
Thanks in advance.
Hi Carvalho,
Sorry it took me a bit to respond to your question, but I wanted to get some feed back from our Lab Director. This is what he has to say:
“This is a very good question and the answer is quite simply, we do not know which Allele was given by mother but we can take this uncertainty into account in our statistical calculations. To explain, we need take a step back and understand how we account for such events such that we can arrive at a Probability of Paternity. To begin, each DNA marker that is used in a paternity test is given a Paternity Index (PI) which is a calculated value based upon the inheritance scenario and genotypes that exist. The PI reflects the weight of the statistical strength of genetic association given the discovered Mother, Child or Alleged Father genotypes. Broken down, the PI is simply a ratio of the test of two different hypotheses: Numerator (the probability that the Alleged Father of a certain race is the Child’s true biological father given the genetic evidence seen) vs. Denominator (probability that a random man of a particular race could be the biological father of the child given the genetic evidence seen). PI must be calculated for each genetic marker (CSF1PO, Penta D, Penta E, FGA, etc.). All PI’s are then multiplied together to get a Combined Paternity Index (CPI). The CPI is then converted to a probability of paternity (such as 99.99%). These are the highest-level of calculation steps that are performed.”
I added a page to the site that walks though your scenario in in great detail. You can view it here:
Understanding Paternity Index
Dear Kate,
Thanks for your answer. I put this question, because i made a paternity teste with my two childs, they are twins (not identical) and their mother. One of them give 99,9999% (child 1) and the other 99,99999% (child 2)so i was curious to know why was the diference. Even because the one(child 1) wich get 99,9999% is the one that is more identical to me. Both of them had that scenario in Penta D (mother has in a locci (Penta D) the alleles 9-10 and the child has in the same locci (Penta D) 9-10) but only child 1 had same scenario in D5S818, that’s why the probability of paternity was lower correct?
I had a match with both childs at all 15 locations. So even with this scenario i have no reasons to assume that there is anyone else (even related) that can be the father of them, because according to your experience only the true father will macth all 15 markers, even with this scenario.
Thanks in advance.
Best regards.
Dear Kate,
I’m adopted and I’ve been contacted by a young lady who thinks I’m her sister. Is there a way to do DNA on the both of us to see if we’re related? We possibly have the same father. However, he passed away this week so we need to do the next step to find out. Thanks for your help.
I have a question. I had a half sibling DNA test done with a girl who I thought was my half sister.-same dad different mothers. It came out that they didn’t think we were half siblings according to the test. Is this the final answer? Is this test totally conclusive? It was like 59:1 or something like that. I have the copy of the results, but I just don’t understand it. Thanks.
Hi Sharon,
The test you are looking for is a siblingship test. This test could be performed on you and the young lady to determine if you are half-sisters (share one parent) or are un-related. Only you and the young woman would need to participate in the test. It’s recommended to include DNA samples from both of your mothers, but it’s not necessary.
You can read more about it here: http://www.dnatesting.com/more-dna-tests/sibling.php
can you take a test with you uncle to see if you have same father meaning my grandfather 2 o positives can not make an a- and my father is past and 1 uncle is past he was a+ and grandfather was ab- and aunt was o+ the uncle that i have living is a b these are all my father brother and sister and my grandma was o+
DNA paternity testing is done by comparing the loci (or markers) on sections of your DNA, not with blood type. While blood type can sometimes provide clues to paternity it won’t give you the definitive answer that DNA testing will.
You don’t say which relatives are available for testing, so I would encourage you to call one of our test consultants to discuss your individual situation.
Kate….My daughters father passed away before paternity was established can his half sister give accurate dna to establish paternity?
Hi Kristi,
We can certainly do a DNA test to establish paternity with a half-sister. These type of tests we call “family reconstruction” tests, and they are a little more complex than a standard paternity test. With this type of test we are trying to “reconstruct” or determine what the alleged father’s DNA looks like, by using the DNA of his known relatives.
You don’t mention if your daughter’s grandparents are available for testing. If either of them are available, that would really help. If he had another sibling to test, that would also be helpful.
In short, I’d encourage you to call one of our test consultants. They would be able to talk to your about your particular situation so that you get the strongest possible result. You can call 1-888-404-4363 or send me an email at kate@identigene.com
Hello,
I had a DNA test taken to prove my father was my father when i was 10 (13 yrs ago). My sister (21 now) never had the opportunity to have a DNA test done to find out who her father is. My mother insists that my father ( we also have heard ‘rumors’ from other family members that it may be his cousin) is also my sisters father, and we have been trying to get him to take the test (even through a lawyer and all of that) but have been unsuccessful with getting his cooperation. My sister desperatly wants to know if this man is in fact her father. My question is…If my sister and I (share the same mother) took a DNA test, what are the chances of being able to tell if we share the same father also?
Hi Beth!
You are in luck! The test you want, is a siblingship test (full vs half). In tests like this it is especially helpful to include your mother’s sample. Including your mother in the test will strengthen your results. If you have further questions, encourage you to call me (or any of our test consultants) at 1-888-404-GENE and we can give you more advice on your particular situation.
Dear Kay,
My husband when he was 17/18 got two girls pregnant within a three month period. Boy 1 born 10/8/87 boy 2 born 3/24/88 (boy 2 birth date is approx not quite sure exact birthday) any way boy 2 was just now told that there is a chance that my husbands brother could be the father. My husband was tested way back when 2 was born and the results were 99.9% but is this new findings of the brother being a possability of paterinty? All are still alive what are the needed steps to find the true parent. After all these years my hubby has moved on gotten married and was just told this. He had dna done in the first place in 88 paid a lot of child support and medical bills for boy 2 and now she is telling her son my hubby might not be his true father but his uncle is. My husband is torn but would like to find out the truth and so would boy 2. Please and thank you for any help you can provide.
Kristen
Dear Kristen,
Thank you for contacting Identigene. When two alleged fathers are brothers, difinitive results are still possible. We always recommend testing both alleged fathers with the child. We also recommend testing the child’s mother if she is available. This does help in the testing process. If only one brother is available for testing you want to notify the lab a head of time.
I am adopted and found both of my parents many years ago. I never had a DNA test with my father, but he said the story lined up and we let it go at that. He has now passed, and I have been told that my birth mother told him he was not my father. There is no way she will agree to a dna test, but I still need to know about my father. One of his children, my believed-to-be half-sister, is willing to do the dna test. His sister, my supposed aunt, is also a possibility. Who would it be best for me to take the dna test with … my half-sister or aunt? Thank you.
Hi Tracy,
When the alleged father is not available for testing, you can choose the sibling DNA test or the aunt/uncle DNA test. Both of these tests will give a probability percentage, and are not as conclusive as a paternity test. However, out of the two I would recommend the sibling DNA test. For best results I would also recommend including her biological mother if she is available. You stated your Mom will not participate, however, we ask for as many biological mothers as possible to test as well.
Hi My name is nadia I’am from the Bronx, NY
We are 3 sister from there mother and one father, my father DNA is very
strong, everyone in the family look like in something, but one of my sister
is completely different of us, we have some doubt because her mother used to be married when she got with my dad, I’m the oldest one and since my father is not around we want to find out if she is our sister.
Can we do DNA between sister a least two of us, since both of us can to USA with a DNA test throughout Immigration department. and where can we get the DNA kit for that type of test.
Thank u
Hi Nadi,
Yes, we offer Sibling DNA testing. The best option would be to test all 3 sisters with any of the Mothers that are available. This is a more complex testing process. I would recommend contacting our client support team (they are wonderful and very helpful) at 888-404-4363 Mon-Fri 9am-8pm EST. They would be happy to assist you with getting everything set up.
I would like to do a sibling test with my brother. Our Mother passed away two years ago, is there any chance that there may be any of her dna available amongst her belongings ie toothbrush, hairbrush.
thank you for your time
Hi Tania,
You can do a sibling DNA test with you and your brother to determine if there is a biological relationship. The best option would be to include any other full siblings you or your brother may as well. There may be a chance of your Mother’s DNA on certain objects, however, they are more expensive to test and there is no guarantee a profile will be found.
Hi..i have a funny but very important question.. If a dna test is done to prove father and daughter paternity. but the portential father is already a half brother to the daughter same dad different mom.. how can the DNA test determine if the half brother is the dad… i guest my question is if you are doing a pertenity test. For someone who already related as half brother and sister how can a dna show a brother is the dad.. its complicate.. i hope you take the time to answer my question