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Autosomal advice please

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  • Autosomal advice please

    In the past day I've been able to do a bit of research on sibling testing and have learned a bit from the related posts on this forum... which I don't want to piggyback on. I am asking for some guidance regarding making sure I am proceeding in the right direction and any advice is much appreciated.

    Situation: I am a female who has finally found my birth family. I have a full brother and a half sister. My birth mother passed away 3 weeks ago before I got to meet her. The presumed birth father (he is on my birth certificate and would have had to sign on my adoption) denies all knowledge of me and apparently would like to keep it that way. I would like to attempt to prove or disprove this for myself.

    It is highly doubtful that I would be able to obtain any forensic DNA from my birth mother. Obviously, obtaining DNA from the birth father is not likely, and asking the relatives on that side is something that would be problematic at the very least, at least at this point.

    I have learned here that autosomal testing is what is in order, but with a female and male there is a high chance of false 'negatives'. If we include our half sister on this test, as I would like to anyway, will that be helpful in my brother's and my chances for determining full siblingship? And would my half sister be the most significant person we could include on the maternal side, or would an aunt be better? Again, it seems that at this point, I will have no one from the paternal side to contribute.

    What would you do?

    Thanks in advance folks.

  • #2

    Do you want to prove by DNA that your presumptive brother is your brother and presumptive half-sister your half-sister and, thereby, that father of record is your father?

    Is your half-sister the daughter of your mother and a father other than the father of record, or a daughter of the father of record and another mother?


    • #3

      Thank you for your reply.

      Exactly. I am trying to prove by DNA that my presumptive full brother and myself have the same biological father (which I can really only assume is the father of record), and that both us and my half sister all share the same biological mother (which is not in dispute but would just be nice to verify for myself).

      My presumptive half sister is the daughter of my mother and a father other than the father of record. I also figured that in testing, adding more maternal DNA to the mix might be helpful on some level, since it appears we won't be able to get paternal.

      Thanks much again.


      • #4

        So, proving a full-sib relationship to your brother would prove the father because the father does not dispute his paternity of your brother?

        1) If all chldren had the same mother all will have exactly the same mitochondrial DNA. So you could do a test of all three children for mt-DNA through FTDNA for $189. per child. An mt-DNA test of HVR 1 & 2 would nail common maternity. And that could be good enough if the paperwork is good.

        2) If the common parent was the mother she gave all three of you a single X chromosome that was a recombined version of the two X's she got from her parents. Each of you likely got a slighly different version and it could be difficult to to match-up except for the three linked markers (DXS 10074, 10075, and 10079) that always descend in a haploblock. You mother had only two versions of the haploblock and if you tested all three sibs, two would have to match, at the minimum. If you are lucky, one of the daughters would match the son and that could be enough if the paperwork is good. The cost is $185. per child

        3) Then there is the autosmal option.You and your presumptive full brother should be tested for both of FTDNA's Autosomal Panels at a cost of $257. per child. You ought to match 50% (the actual range of match of two full siblings is 0% to 100% but the average is 50% but I have tested myself and 5 siblings and found the average to be reliable). Autosomal is not perfect (I match 50% to a 2000 year old Mongolian burial at Eygin Gol) but taken in conjunction with one of the other tests above ought to prove convincing.

        BTW, Dr. Thomas Krahn at FTDNA is an expert in this stuff, so you might consult him. He will render a professional opinion of relatedness based on the test outcomes.

        Hope this helps.


        • #5

          This is extremely helpful and I thank you for your detailed reply. I think we are going to start with the autosomal for all 3 of us and hope that we get 'lucky' with the results. Unfortunately, finances won't allow me to get the panels that I wanted to get here... but I did find what looks like a good starting point with a 16 allele sibling index elsewhere that uses CODIS and STR. I just hope that compares somewhat with the Autosomal 1 and 2 panel test with the 9 CODIS markers+high resolution STR markers that I would really have preferred.

          So hopefully out of this we will get percentages close enough to know if we can rest easy, or need to do some additional testing. The birth father does not dispute paternity of my presumptive full brother... so I'm hoping to see that 50% for my brother and myself with the expected 25% for both of us with our half sister.

          But if I match up with any 2000 year old Mongolians... you will be the first to know! That's pretty interesting btw, how would that be explained?

          Thanks so much


          • #6
            Unfortunately my match to Eygin Gol Mongolians from 2000 years ago, demonstrates the weakness of autosomal STR's in establishing relationship. The atSTR's are rather 'cosmopolitan'.

            In forensics they are usually used when there is a sample that can be matched-to, a paternal sample, crime-scene sample etc. Absent such a sample one can only improve their discriminating power by testing a lot of them.

            In your case, with no paternal sample, the best you can do is test around the question of paternity in support of public records that indicate paternity such as demonstrating common maternity.


            • #7
              X Test

              I was also adopted and tested to prove paternity. My father was deceased.I had a sister on father's side to X test with

              There is no exact test to prove you and your brother share the same father by you taking a test together.

              The X test or paternity test would be the only accurate way but since your father wont test, you will need a female to test. you would either have to share a sister with the same father or you could test with your grandmother ( father's mother ) because you carry her X chomosome

              Your father passed his Y chomosome down to your brother and his X chomosome down to you. You do not carry the Y and your brother does not carry the X. Here is a thread I started about X Testing and siblingship