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I found a 1st cousin not connected to my mom and my dad was an only child. She has 1053 cM on 40 DNA segments. I have heard people say 1st cousins can also be half-siblings?

If she is a first cousin and my dad had a half-sibling he did not know, would her cm be that high and if so how could I find this aunt/or uncle if he doesn't know we exist?

I am female..my mom did a DNA test and she does not match with this cousin. My Dad is no longer living.

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    How exciting! Could you add where each of you was tested, whether you are sharing information, and your relative ages? It would also be useful to know how much X-DNA matches. You can find this out by uploading the data to gedmatch f(free) if your current site doesn't provide that information. Good luck! – Ellen Spertus Jul 14 '18 at 8:01
  • Hi thank you. We tested through ancestry.com I dont know about shared data? – Shasha Jul 14 '18 at 20:20
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    Could you update the post with the information in your comment? – Ellen Spertus Jul 14 '18 at 21:34
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Possible relationships

According to the Shared cM Project, a woman with 1053 shared cM could be your:

  • First cousin (553-1225)
  • Half aunt/niece (500-1446)
  • Great aunt/niece (251-2108)

I have left out some relationships, such as great-grandchild, which seem unlikely.

Data from the DNA Detectives predicts the same relationships, giving all a range of 575-1330 cM.

Data from both sources make it extremely unlikely that your match is a half-sibling.

Further research

Because you took AncestryDNA tests, here are some next steps:

  • When viewing the match at AncestryDNA, click on "Shared Matches". That will tell you who else matches both you and your original match. You may be able to use this to tell if she's related to your father's mother or father's father, etc. (Even though their DNA isn't on Ancestry, the DNA of people related to one but not the other of them presumably is. If not, ask a 2nd cousin on your father's side to take a test.)

  • You can download your DNA from Ancestry and upload it to GEDmatch. If your match does the same, you can get more information on your DNA match there, including how much your X-chromosomes match. You can read about how to interpret that information or ask us in a new question.

Good luck!

  • So maybe this is a sibling of my Dads he did not know? That would be cool too :) – Shasha Jul 14 '18 at 20:22
  • @RobertShaw Thank you. I've updated the post. – Ellen Spertus Jul 14 '18 at 21:33
  • @Shasha Yes, your father having another half-sibling is a possibility. It would be helpful if you would update your original question to answer the questions in my first comment, including whether you are in communication with your mystery match. (Ancestry lets you send messages to matches.) – Ellen Spertus Jul 15 '18 at 20:14
  • Im new to this and I dont see where to go to update the question.. This mystery match has not responded to my messages but it shows the last sign on date to be in May so I'm thinking she has not seen it :( – Shasha Jul 16 '18 at 15:33
  • @Shasha How frustrating that they haven't answered! I believe messages are typically emailed to people, so the recipient may be ambivalent or have missed it. You can update your question by clicking on the "edit" link under the tabs. – Ellen Spertus Jul 16 '18 at 17:14
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Here's a link to a visual representation of how someone sharing 1053 cM with you could be related: Shared cM Project tool for 1053 cM

The potential relationships, and how that connects her to you:

  • First Cousin: your dad would have an unknown full sibling, not half. (You and she share too much DNA to be half-first cousins.)
  • Half-Aunt: she would be an unknown half-sister of your father's.
  • Great-Aunt: she is a sibling to one of your dad's parents.
  • Half-Niece: you have an unknown paternal half-sibling, who would be one of her parents.

Get in touch with her; you've got new family! :) As always, tread carefully; she may have just learned some things about her family that she didn't know.

  • Thank you so much everyone. This person has not yet responded to me. I see her last loggin was may 31 so she may not have seen my message. I'm so curious can hardly st and it :) – Shasha Jul 15 '18 at 16:33

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