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Brother and Sister, different father?

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  • Brother and Sister, different father?

    My sister believes that she was fathered by someone other than our dad, and we are curious about how to go about determining wheather or not this could be the case.

    What tests can we take to determine wheather we share the same father?

    -gk

  • #2
    The Family Finder test will clearly show whether you and your sister are full or half siblings. This is based on how much DNA you share in large segments. Full siblings share significantly more DNA than half siblings do.

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    • #3
      Agreed. It's not subtle at all if you know what to look for.

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      • #4
        For half the cost you could test at 23andMe and get the same answer. Best of luck to you both.

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        • #5
          23andMe will tell you a half-sibling is an aunt or uncle (at least that's my experience); I haven't figured out how to correct the designation. FTDNA lists half-siblings as one of the possibilities. Of course the amount of shared DNA is comparable for half-siblings and aunts/uncles.

          Originally posted by Kasandra View Post
          For half the cost you could test at 23andMe and get the same answer. Best of luck to you both.

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          • #6
            A little OT: In my case, this may be due to the age difference, as my half sister is projected to be my aunt and I her nephew. If so, it's a pretty foolish approach; it's not all that unusual for half siblings to be a couple of decades apart in age, and it's not unheard of for uncles/aunts to be younger than nieces and nephews.

            Originally posted by NYMark View Post
            23andMe will tell you a half-sibling is an aunt or uncle (at least that's my experience); I haven't figured out how to correct the designation. FTDNA lists half-siblings as one of the possibilities. Of course the amount of shared DNA is comparable for half-siblings and aunts/uncles.

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            • #7
              It will be easier to see if a full sibling or half sibling relationship through 23andme. In their chromosome browser you will get a good image if you are related through one parent or two. There will be some overlapping or "Fully Identical" segments which will appear dark blue in full siblings. Half siblings who are only related through one parent will not have these and will only show light blue shared segments (full siblings will show a combination of dark blue segments mixed with light blue segments). FTDNA doesn't show this, although you can upload your raw data to a 3rd party site that will do essentially the same thing. It is cheaper and probably better in your case though to test at 23andme, you can always transfer your results later to FTDNA if you wish.

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              • #8
                Originally posted by NYMark View Post
                23andMe will tell you a half-sibling is an aunt or uncle (at least that's my experience); I haven't figured out how to correct the designation. FTDNA lists half-siblings as one of the possibilities. Of course the amount of shared DNA is comparable for half-siblings and aunts/uncles.
                23andMe does not tell you anything. It predicts a relationship based on the amount of shared DNA and the ages of the two people being matched.

                That said, the important information is not the predicted relationship; the important information is the amount of shared DNA. With this, a brain, and a little work, you may be able to determine the true nature of the family relationship.

                Whether the status of your match is private, Public Match, or Sharing Genomes, you may change your match from its predicted relationship to its known relationship. Simply click on the sharing status in DNA Relatives, formerly Relative Finder. A dialog box will popup will appear that allows you to make the change.

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                • #9
                  Thanks for the tip on how to correct the prediction. I'll have to see if it works, since I'm managing the account.

                  As to the latter, sorry my language wasn't clear in the first post (it was in the follow-up.) FTDNA predicts a range of possibilities: half sibling, uncle, grandparent, while 23andMe only provided one prediction, something I'd still contend is a bad idea. As an aside, when I compare their predictions of relatedness for my fully AJ half sister and me, they're just about as woefully off-base as FTDNA's projections for my half-brother and me.

                  On edit: it's actually not an option, since the sharing status column reads "Owned Profile" and there's no clickable link. Doubly stupid.

                  Originally posted by MFWare View Post
                  23andMe does not tell you anything. It predicts a relationship based on the amount of shared DNA and the ages of the two people being matched.

                  That said, the important information is not the predicted relationship; the important information is the amount of shared DNA. With this, a brain, and a little work, you may be able to determine the true nature of the family relationship.

                  Whether the status of your match is private, Public Match, or Sharing Genomes, you may change your match from its predicted relationship to its known relationship. Simply click on the sharing status in DNA Relatives, formerly Relative Finder. A dialog box will popup will appear that allows you to make the change.
                  Last edited by NYMark; 14 April 2013, 10:29 AM.

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                  • #10
                    Originally posted by NYMark View Post
                    On edit: it's actually not an option, since the sharing status column reads "Owned Profile" and there's no clickable link. Doubly stupid.
                    Actually, you can click anywhere in the row that is NOT already a clickable link (e.g. profile name, haplogroup). It brings up a dialog box where you can specify the relationship and also enter notes (which become searchable). I keep telling 23andMe that people aren't aware of this feature.

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                    • #11
                      Thanks very much! You're right; it's totally counterintuitive.

                      Originally posted by Ann Turner View Post
                      Actually, you can click anywhere in the row that is NOT already a clickable link (e.g. profile name, haplogroup). It brings up a dialog box where you can specify the relationship and also enter notes (which become searchable). I keep telling 23andMe that people aren't aware of this feature.

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                      • #12
                        Aunts or uncles share about 25% of their autosomal DNA with a nephew or niece. Half siblings share about 25% of their autosomal DNA. The confusion isn't surprising.

                        Timothy Peterman

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                        • #13
                          I wasn't confused about the designation, and I understand the percentages. My problem was essentially with 23andMe's interface and the presumption that someone who shares ~25% is an aunt or uncle, since there are other possibilities.

                          My next question, which is perhaps better suited to the 23andMe site, is how does one designate a half-sibling on their family tree page? No such option appears to be available.

                          For all the recent criticisms of FTDNA of late (my own included), the user experience at 23andMe is poor, and the new version has made things worse. My big gripe is about my non-Ashkenazi matches, but the site is now very hard to navigate. There are many counterintuitive features and a whole lot of clutter at the top of the new homepage.

                          Originally posted by T E Peterman View Post
                          Aunts or uncles share about 25% of their autosomal DNA with a nephew or niece. Half siblings share about 25% of their autosomal DNA. The confusion isn't surprising.

                          Timothy Peterman

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                          • #14
                            Thanks to all of you

                            I appreciate the advise and comments, it is useful as I decied the best way to go in this case.

                            Thanks to each and everyone of you.

                            -gk

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