Half sibling DNA test is more conclusive with mom

Posted on May 4, 2010

half sibling DNA test If you and someone you know believe you share a father, but you know you do not share a mother, then there are DNA tests that can help prove your theory. Here are your two options:

1. If you are both males, then a Y-chromosome DNA test can prove if you share a common male lineage. The Y-Chrome test does not prove if you have the same father, because all males from the same family share the exact same Y profile. Therefore, you share the same Y-Chrome profile as your father, your brother, your uncle, your grandfather, your son, and so on. With a Y-Chrome test you need to be certain that other family members are not potential fathers.

2. If your half sibling is a female and you are a male, or vice versa then a half vs. unrelated siblingship test may be right for you. Depending on your genetic makeup, the half sibling test can be extremely conclusive. There is also a chance that the results are fairly inconclusive. A situation where results would be very conclusive is when you and  your half sibling share very rare genetic markers. Since this is unlikely to happen, the probability of you being related is much higher.

One way to improve the conclusiveness of a half sibling DNA test is by including one or both mothers. This allows the DNA technicians to exclude the half of your DNA that is contributed from your mom, and isolate the half that was contributed by your father. Once it is isolated, it is easier to get a more conclusive result.

Our laboratory guarantees DNA test  results that are greater than 99.9% accurate for all paternity tests. Although we do not make the same guarantee for half sibling tests, there are many things we can do to improve the conclusiveness of your results.

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  • steve fiore says:

    I am interest in having a half sibling dna match performed
    the match will be male /female.
    female mother not cooperative to supply her dna
    i will try to work on that
    like to know if a male son survivor of father’s brother
    would make a difference?
    steve fiore

    • Ryan Q says:

      @steve do you believe that the potential half siblings share a mother or a father? If you believe they share a father, then a son of the common father could be used in a Y-chromosome test. This would be conclusive in proving whether the male sibling is biologically related to the father. If you have more questions please feel free to call us. Our case managers would be happy to speak with you. – Ryan

  • steve fiore says:

    would a male first cousin to the female half sibling be able to
    increase the percentage of a match if his dna was submitted?

  • Lea says:

    My father recently passed away and I believe that someone who has been known to me as a second cousin may actually be my half-brother. What are the chances of a DNA test telling us anything since we already know we’re related?

    • Ryan Q says:


      If we include both your mother and your potential half brother’s mom, there is a good chance we could get a conclusive result telling you if he is actually your half brother.


  • jonathan morris says:

    Have already had tests done for half siblingship with two alleged sisters and their mother.(Definitely not my mother. She is unknown.) Alleged father is deceased. Many of the alleles match up, (if I am understanding it correctly), but some of the alleles from the sisters and mother don’t match. So it makes me wonder if I’m not understanding it correctly. The test says “the sibling index for half siblings is 0 to 1, but there are exclusions in 3 of the 20 systems”. The mother tested, for whatever reason, believes the alleged father is my father. Could someone please help me understand this?

    Thank you,
    Jonathan Morris

  • Tammie says:

    Hi my biological father is not living I recently have been informed he had a son with another woman to be my half brother – the half brother in questions mother is alive for testing however my mother is not can the half sibling test for same father be accurate with only one mother available?

    • Ryan Q says:


      Thanks for your question. The results can be accurate and conclusive. We never know until we run the testing, as it depends on your unique genetic makeup. Contact me if you have more questions.


  • Gloria says:

    Is it possible to see if a male and female are half-siblings by the female’s father, if these two “half-siblings” are known second cousins through the female’s mother? (her mother is first cousin to mother of male “possible”half-sibling.
    No parents are available for testing.

    • Ryan Q says:


      We can do a half siblingship test. However, without testing the parents, there is a risk of getting inconclusive results. Give us a call if you would like more information.


  • Lily says:

    What if you suspect to be sibling with your cousin. Siblings would be female/male, with same father, different mother but the mother is the other siblings aunt.

    • Maria Soto says:


      I would suggest that if the potential father is available, to include him in the testing. Any time, that potential siblings are already related, it may affect the conclusiveness of the results. In these particular cases, it’s important to include the potential father or mother, to achieve the best results possible. I would love to hear about your particular case and assist you. You can always contact us directly over the phone so we can assist you the best way possible! Thanks for your interest in our company and our blog!

      Maria Soto
      DNA Case Manager

  • Rev says:

    Recently a couple of relatives submitted DNA swabs for analysis with another DNA company. I am not questioning the results but I would like clarification about the meaning of the interpretation. I’ve received a copy of the test report and the following was the Interpretation:

    Combined Siblingship Index: 536
    DNA testing was done to determine siblingship of the alleged siblings. Based on testing results obtained from analyses of the DNA loci listed, the probability of half-siblingship is 99.8%. The likelihood that they share a common biological parent is 536 to 1. This probability of siblingship is calculated by comparing to an untested, unrelated, random individual of the Caucasian population (assumes prior probability equals 0.50).

    I would like clarification about the following sentence:

    “Based on testing results obtained from analyses of the DNA loci listed, the probability of half-siblingship is 99.8%.”

    Is this sentence saying the two are only half-siblings?


    Could this sentence communicate that the two are at least half-siblings? (With the understanding that full siblingship could be determined by further testing.)

    I would appreciate any assistance in helping me understand what the test results communicate.

    • Rev says:

      The two relatives were male.

    • Maria Soto says:

      To whom it may concern:

      The results in this case mean that the two relatives are half-siblings. Siblingships rely mostly on the probability percentage and also the likelihood. In this case the probability is extremely high, it’s the maximum (no DNA test will have a probability of 100%, the highest possible is a 99%) and also the results state that 536 times in 1 they share a biological parent. The probability is the highest which means they are related and are half-siblings.So, just to clarify, the results indicate that they are half-siblings and they do share a common biological parent. If they weren’t related the sentence would say: The likelihood that they do not share a common biological parent is ___ to 1. I hope all of this iinformation is helpful to you. Thank you for your interest in our company.

      Maria Soto
      DNA Case Manager

      • Rev says:

        I do appreciate the reply, but I think I need more clarification. Can you determine from the information provided that they are by no means full siblings and that they absolutely do not share two biological parents? I do have the breakdown of the X and Y lines that one may need to make that determination. Would such a test indicate whether they share a father or a mother? Or would a company need a DNA test of the mother in order to determine anything more definitively.
        Also, if your company was provided the data from a previous test, could a test be run on the mother to determine whether the two individuals share both parents or would your company need fresh tests from the two men and the mother?

        • Maria Soto says:


          I’m not familiar with your particular case, and I’m not sure with which company you tested. At least with Boston Paternity, anytime a Siblingship test is done, we always ask if we are trying to prove a half-siblingship (and on whose side: paternal or maternal), or if it’s a full siblingship. In this case it clearly states that the test was a half-siblingship so I understand that the potential siblings, or whomever started the process, notified the DNA Testing company that it was a test to prove half-siblingship. A half-siblingship test tries to determine if the two tested individuals are likely to be related or not. No siblingship test can prove 100% that you are related, it only states a likelihood based on a statistical chance. However, if the two individuals were certain that they were related already and just wanted to prove the likelihood of being full siblings or not, this should’ve been communicated to the lab due to the fact that full-siblingship tests determine the likelihood that they are full siblings or not. It is important to note that it is not possible to determine with 100% certainty whether or not two people are definitely siblings; only whether they are likely or not to be siblings. To answer your second question, if I want to do a half-siblingship, I’m clear that I understand that I may or may not be related to this person on my mother’s side or on my father’s side. I would need to tell my lab that I just want to see if I’m related to this person by my mothers side or not, seeing that it will determine the likelihood of a half-siblingship. If you want to prove 100% that you are related by a mother, without any likelihood involved, you would need to include the mother or father in question in the testing, which would be by doing a maternity test or paternity test. The mother or father in question would have to get collected, and their sample would have to be included in the testing for it to be a definitive conclusive result. To answer your third question, we can’t re-use samples that were previously used with another DNA testing company. We can only re-use samples that have been tested with us for legal DNA tests, and with additional authorization to do so. In this particular case, I would suggest that you reach out to the company that tested these individuals. They should be able to give you the answers you need according to your specific case. I can’t base my facts to another companies DNA test results, because it’s relative to how the lab processed the results, and not all labs have the same procedures in testing. I hope all this information is helpful to you, and should you have any questions, you can always contact us at 1-877-436-9323.

          Maria Soto
          DNA Case Manager

  • MJ says:

    My mother believes she may have been fathered by another man. How would I go about finding out who my grandpa was when both possible fathers are deceased? She did have 9 siblings though. And the possible father’s children are still alive (but not likely to help since the possible father had money). Can my mother have a DNA test done with her brother or sister to find out if they are half or whole siblings?

    • Maria Soto says:


      Since both potential fathers are deceased, the best next thing would be to test the brothers of the potential fathers. Since we would have to reconstruct the DNA of the possible fathers, this would be a great option. However, that would be done of course if the possible fathers’s brothers or sisters are alive if not she can do a Siblingship test to determine if she is a full-sibling or half-sibling. So to answer your question, she definitely can do a test with her siblings. I would always suggest trying to do an Avuncular first (test to prove relationship between an aunt/uncle and a niece/nephew), and if that isn’t an option she can do a Siblingship.

  • J.C. says:

    I have a problem with my husband not wanting to do a dna test done on our oldest son, but I want to know if he is the real father. How would I go about doing a dna test without him knowing? My youngest son said he will help out to find the truth, I do know he is my husbands true son. Can I do a dna test with both of my sons, and without their father knowing?

  • Katie says:

    My mom believes she was raped while drugged when she concieved me. His record was terrible, but this particular case was never proven although my mother had her current boyfriend tested which was the only other man she had been with. We have very well believed that he was my father ever since then, but we have never had a paternity test to prove it. He has been dead for almost 9 years now but my sister is still living with her mother and I know my grandmother. We contacted my believed to be sister and she refuses to take a paternity test or to even believe her father would do such a thing. She is 19 and I am 16 and both of our mothers are still living. Could we legally now get a paternity test by law?

  • Kathleen says:

    My sister and I want to know if we are full siblings or only share the same father and not the same mother.
    Both mother and father who raised us are deceased, so no parental DNA is available.
    Two years ago we did siblingship DNA using saliva method.
    The report findings read as follows:”Based upon the statistical analysis….it is 99.997% that tested Sibling 1 and Tested Sibling 2 are full biological siblings versus not being related at all.”
    It doesn’t mention half siblingship, which leads me to believe that what is being compared is that it is 99% we are related in some way, at least sharing one parent, than none at all.
    I have contacted company numerous times, to no avail, and now wonder if other testing might be needed to find our answer regarding half versus full siblingship.
    Thank you for your response.

  • Nicole Carlson says:

    My sister and I have the same mother,but thereis some doubt on whether we have the same father.We are looking in to getting tested,but there is no way our mother would cooperate and give a sample.We do not have any contact with the alleged biological father. How accurate would the test be to determine if we are full siblings.?

    • Maria Soto says:

      Hi Nicole:

      Please give one of our Case Managers a call, at 1-800-362-3588.

      • Biki says:

        Hello Everyone, I really need your help and advice to identify the case, I have a half brother born from the same father and we would like to check whether we are related or not using half sibling test but the problem is both of our mothers and our father are died so how can we proof and get a correct analysis?
        please help ne I really need to have a brother!?

  • Brenda says:

    Over the last 18 months my siblings have had 2 DNA tests done with a woman who turned up after our Father had died to state that she was his daughter. The first DNA was done with my sister and the CLR came back at 1.73 with a probability of relatedness of 63.40% (inconclusive). The second DNA was done with my brother which brought back a CLR of 0.11 and probability of Relatedness of 10.53%. The potential half sibling is now kicking off big style because the result dropped so low.

    Are we as a family right to believe that she is not our fathers daughter (Half Sibling) even though the results are inconclusive or do we need to do another test? We are struggling to understand the result and the DNA centre we had them done at are not that helpful.

  • Veronica says:

    Hello. My mother believes her aunt, her mother’s sister, may have actually been her mother. Both her aunt and her mother are deceased. Is it possible to test my mom and one of her cousins (who may actually be a sibling) to determine the possibility of her aunt being her mother? I was thinking the best test may be to do a half-siblingship test to determine if her cousins are actually siblings. That would give her the same answer. Thank you.

  • KJ says:

    My husband died and is cremated. We have son and daughter and my husband has a brother. Another women claims to have my husband child. Can the people available for the sibling DNA testing provide the combination that gives the undeniable results – 100% certainty.
    Uncle and the alleged child?
    My son or daughter and alleged child?
    Alleged child his mother and my son or daughter?

  • jacki says:

    I took a siblingship test with my mother, my sister(share mom) and woman(believed to my sister) (she also shared the same dad with my sister with whom I share same mom) We used my sister as a known daughter of deceased dad, along with deceased dad’s daughter. the results came back 99.96% that I was full blooded with sister and half sibling with woman. Is this test conclusive or a false positive? Can a known sibling be used in place of father’s DNA?

  • Champion says:

    My younger sister and brother are twins. We have the same mom. My older brother had a different dad than the twins I was born in the middle. My dad is supposed to be the twins dad. I ordered a DNA test to test to see if the twins are my full sibling. I’m waiting for it to come in the mail. If it comes back half should I test my dee brother for full sibling. His dad is dad the twins dad is unspeakable and my mom I think wants to believe the twins and I have same dad and won’t utter a peep. Will this test be accurate as I am testing twins how does that work

  • Brooks says:

    I did a y chromosome test and it came up 0% but I did a full siblings test and it came out 1.1. I read that if it is over 1.0 they are full siblings. Can you help

  • Jessica says:

    My father passed away and we resently found out we (my full brother and sister) could have a half brother from our father. The other possible father is my dads cousin on his mothers side. What kind of test could we do?

  • Shannon says:

    PLEASE HELP! I had a siblingship test done on my two sons. The representative did bucchal swabs on one day on the boys but contacted me days later to take mine as he had forgotten. The results were not as I expected as I was certain of the time I was with my younger sons father. There seemed to have been 16 loci tested. The results showed exact number matches on 6 out of 13 loci on allele A and on 8 out of 13 on allele B. The results also state a combined likelihood ratio of 215.9 and probability of relatedness as full siblings as 99.539%. When I called the company for clarification they claim the results are certain though they never received my sample. Should I also have had my daughter tested? How accurate are these results if they never considered my sample? If the two boys are indeed full siblings why are there not more matches? I thought males have near identical matches to their father.

  • Miranda says:

    Ok so this is my question: We know for a fact my husband has a son with his ex their was a DNA
    Test done on him and she is claiming her daughter is my husbands now the mom won’t give up her DNA this time how do we find out if he is the dad what test do we do?

  • Kirsten says:

    Hi me and my cousin r both females and both r parents have passed away but it has come to r attention that my father had an affair with her mother and we could be half sisters how would we go about having a test done

  • Lisa says:


    I would like to have a half sibling test performed; female/female that possibly share the same father. The father is not available for testing but both mother’s are available. Can a test be accurately performed this way?

  • Shirley A. Miner says:

    I was informed that my Mother’s name was on a birth certificate of a man I never met and lives in Ca. I in Ma., now Fl.. He was adopted when he was 3 months old. I did an DNA test from Ancestry.com and it came back I was a first cousin. Could this mean I am his half sister. We do not have the name of his father.

  • Leslie says:

    Which is more conclusive/decisive: an avuncular test with aunt/uncle of alleged father, or half-sibling test (I am a female and have what I believe are 3 female half-siblings). Thank you!

  • Elizabeth says:

    How accurate will a sibling DNA test be if trying to determine paternal father if using deffinate mothers sample and sister to sister siblings? Would brother to sister sibling be more acurate? What if possible father are brothers and am not able to get sample from either.

  • Barbara says:

    I am looking for help to know what testing method To to use to find out if my friend could be my sister? Her mother told her that her father was my father. Both father and mother of her side and my side are deceased. Is there anyway that we can be tested to find out if we are sisters if her father is my real father? Thank you for taking the time of reading this. If you can be any help to direct us of what test we can do greatly appreciate any advice. Sincerely Barbara

  • Preston says:

    My partner and I have surrogate children from India who share the same biological mother (at least that is what we were informed of in the process). The boys are very happy, healthy 20 month old boys and we could not be happier!

    Now that we are getting ready to start school, the question of siblingship has come up. Although they are being raised as brothers (regardless of any DNA results) and will someday know their stories, we would like to get siblingship DNA testing done; however, seeing as the bio mom is from India, we do not have the ability to get a sample of her DNA.

    What is the best option for us?

  • angel guidry says:

    Hi my 3 year old son took a DNA test when he was 6 months old with possible father it came up 0% what are the chances of possible fathers half brother will also be 0%? Note they didn’t swap my month she they did DNA testing

  • Mitzi Rogers says:

    I want to test a male and female child to see the likelihood that they share the same father. The mother of both would be available for testing. What % accuracy could this be done?

  • Lynne says:

    Could half sisters show as full siblings in sibling test?