Are 1,053 shared cM likely to be a cousin or a half sibling?

Question

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.

Answer 1

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!

Answer 2

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.

Answer 3

A female sharing 1053 cM with you could be your:

• First Cousin: The range is 553-1225 cM
• Great-Aunt: The range is 251-2108 cM
• Half-Aunt or Half-Niece: The range is 500-1446 cM
• Great-grandmother: The range is 464-1486 cM
• Double first cousin once removed: Expected to share around 850 cM

As your mother isn't related to the woman, she could not be your great-niece or great-grandchild. But the match could be either of these five I said above (although great-grandmother would be very unlikely).