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Half sisters, same father

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  • Half sisters, same father

    Results came back for half sisters (we think--trying to confirm with DNA) who have the same father, and I was expecting the X chromosome to be an exact match. However it has a couple of gaps in it that are larger than I would expect. Other examples of half sisters I've seen showed a solid X chromosome shared, passed down unchanged from the father.

    For these new results, the total shared autosomal DNA is 1736 cM which is consistent with half siblings. But the X matching is in three blocks as follows:

    X 2812568- 47310712 76.32 cM
    X 53161383- 71344844 7.81 cM
    X 75114910 - 154570039 98.61 cM

    Is this a result of no calls, an abnormality, or a glitch in the system? Is it possible they are not half sisters?

    Thanks.

  • #2
    its a half sibling

    Originally posted by ToddH View Post
    Results came back for half sisters (we think--trying to confirm with DNA) who have the same father, and I was expecting the X chromosome to be an exact match. However it has a couple of gaps in it that are larger than I would expect. Other examples of half sisters I've seen showed a solid X chromosome shared, passed down unchanged from the father.

    For these new results, the total shared autosomal DNA is 1736 cM which is consistent with half siblings. But the X matching is in three blocks as follows:

    X 2812568- 47310712 76.32 cM
    X 53161383- 71344844 7.81 cM
    X 75114910 - 154570039 98.61 cM

    Is this a result of no calls, an abnormality, or a glitch in the system? Is it possible they are not half sisters?

    Thanks.

    Comment


    • #3
      Originally posted by ToddH View Post
      Results came back for half sisters (we think--trying to confirm with DNA) who have the same father, and I was expecting the X chromosome to be an exact match. However it has a couple of gaps in it that are larger than I would expect. Other examples of half sisters I've seen showed a solid X chromosome shared, passed down unchanged from the father.

      For these new results, the total shared autosomal DNA is 1736 cM which is consistent with half siblings. But the X matching is in three blocks as follows:

      X 2812568- 47310712 76.32 cM
      X 53161383- 71344844 7.81 cM
      X 75114910 - 154570039 98.61 cM

      Is this a result of no calls, an abnormality, or a glitch in the system? Is it possible they are not half sisters?

      Thanks.
      Did they test at the same time?
      Depending if one tested on recent chip being used verses a previous version there is a few spots that will not match due to some small variations on how each chip records certain SNP's. it Seems to Records opposite side of DNA Strand ie. Record A on one chip, but T on other, and G for C. Most of these are accounted for by matching algorithm but I have notices a few spots that get over looked,

      A few Build 36 Positions on X where largest non matching occurance in regards to my kits are (Comparing 3 sisters to father)

      122664306 old chip(2010) records T while last two chip versions record A
      125051065 old chip(2010) records C while last two chip versions record G
      126309782 old chip(2010) records G while last two chip versions record C
      126687359 old chip(2010) records G while last two chip versions record C

      My sister to our father are as follows

      Sibling 1 X 1370495 48421735 85.5
      Sibling 1 X 48631670 122474770 52.61
      Sibling 1 X 126995150 132378403 3.85
      Sibling 1 X 132770507 148386632 32.99
      Sibling 1 X 148955136 154570039 13.08

      Sibling 2 X 1370495 126198816 144.66
      Sibling 2 X 126995150 132378403 3.85
      Sibling 2 X 132770507 148386632 32.99
      Sibling 2 X 148955136 154570039 13.08

      Sibling 3 X 1370495 122474770 138.34
      Sibling 3 X 126995150 132378403 3.85
      Sibling 3 X 132770507 148386632 32.99
      Sibling 3 X 148955136 154570039 13.08

      Most, in my opinion of your case, would be the result of non calls and/or chip variances if tested on different chip versions.
      Last edited by prairielad; 21 March 2014, 08:09 PM.

      Comment


      • #4
        Thanks. The tests were just a month or two apart. A total X match would be about 196 cM, but the total between these half sisters is about 183 cM. Seems a little odd.

        Comment


        • #5
          If you have access to the raw data, I'd suggest using David Pike's utility for comparing two files. You can use just the X file if you wish.

          http://www.math.mun.ca/~dapike/FF23utils/pair-comp.php

          Then you can see what's happening "under the hood."

          Comment


          • #6
            Originally posted by Ann Turner View Post
            If you have access to the raw data, I'd suggest using David Pike's utility for comparing two files. You can use just the X file if you wish.

            http://www.math.mun.ca/~dapike/FF23utils/pair-comp.php

            Then you can see what's happening "under the hood."
            FTDNA has been known to smooth out the gaps on the X chromosome if you point out these holes in known relationships. I bet there should be a solid match here unless the relationship could be through the mother. If you know it has to be through the father, then an error in matching has occurred.

            Comment


            • #7
              It has to be from the father. They know who their mothers are. One has a Cuban mother who contributed substantial Native American and African DNA in the one sister's Population Finder, while the other sister has neither of those ethnic results.

              Thanks Kathy for pointing that out. I don't have access to one of the kits but I will forward that URL to the sisters.

              Comment


              • #8
                I just remembered-- one of the sisters transferred her raw data to FTDNA from Ancestry.com. Could this have an effect? The other tested directly in FTDNA.

                Comment


                • #9
                  Originally posted by ToddH View Post
                  I just remembered-- one of the sisters transferred her raw data to FTDNA from Ancestry.com. Could this have an effect? The other tested directly in FTDNA.
                  Yes, I think that could be the explanation. My son and I have significant gaps on the X. It's actually a double transfer, one from AncestryDNA and the other from 23andMe. We do match exactly at 23andMe.

                  Unfortunately, the Chromosome Browser is down for maintenance right now, so I can't post a screen capture. I'd be curious to see if the gaps are in the same place.

                  Comment


                  • #10
                    Originally posted by Ann Turner View Post
                    Yes, I think that could be the explanation. My son and I have significant gaps on the X. It's actually a double transfer, one from AncestryDNA and the other from 23andMe. We do match exactly at 23andMe.

                    Unfortunately, the Chromosome Browser is down for maintenance right now, so I can't post a screen capture. I'd be curious to see if the gaps are in the same place.
                    OK, thanks for confirming that you and your son have gaps also as a result of transfers. In my first post above, you can see where the gaps are between these half sisters.

                    Comment


                    • #11
                      In my ftDNA data there are 27 SNPs in the range of the gap above. On my 23andMe data there are 34 SNPs in this range. None of the positions align [a build 36 vs 37 issue. likely], only 11 of the rs-designated markers align; 23andMe has a bunch of [9] proprietary i-designated markers in their list in this range.

                      Not knowing the algorithm details, these are just possible places to look...

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