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I was adopted as a small child and have never known my natural mother or father. I recently completed an AncestryDNA test and have been matched against a lady with a score of 1,912 cM across 73 segments. We have spoken and we are only three years apart in age. Both of her parents have passed away. She was born not far from where I was born in London. Both her mother and father had brothers.

My knowledge of the DNA analysis is limited but it seems to me that we could be half siblings. However, I don’t understand the significance of the 73 segments. Could someone tell me how likely is it that we are in fact half siblings or is it possible that her uncles are the link with me?

  • If you can import your data into GEDmatch, you will be able to compare the X chromosomes, which can help you determine whether she's related on your mother's side or father's side. – Ellen Spertus Jan 17 '19 at 20:51
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1912 cM is an excellent match; this woman is definitely a close relative.

Let's take a look at the Shared cM Project numbers:

  • Grandparent fits the cM range but obviously it's impossible, given your ages.
  • Half sibling fits well (range 1317-2312).
  • Aunt/niece fits well (range 1349-2175).
  • Great aunt also fits (range 251-2108) but we'll rule it out due to age.

Those are your only possibilities. One of the following things is true:

  • One of her parents is also your parent.
  • One of her full siblings is your parent.
  • One of this woman's parents is your full sibling (unlikely but not impossible).

If one of her uncles was your father, this would make you first cousins. The range for first cousin is 553-1225 cM and that just is not close enough to 1912 cM.

We can also rule out that she has a half sibling that is your parent (or that another half-sibling of yours is her parent). Because half aunt has a range of 500-1446.

How can you tell the different between a half sibling and an aunt? You can't. Not without more information. Has anyone else in her family tested (or are they willing to test?)? You want to look at the total cM match between you and any other relative of hers.

You also want to see if you match people known to be on her father's and her mother's side. If you match both sides (and you don't belong to an endogamous group), this is a good clue that you are the child of this match's sibling. If you had a match in the first cousin once removed range with two first cousins of your match (one on her mom's side, one on her dad's), that would solidify that she was your aunt. Also if you had no match at all with one of her first cousins (that she matches as expected), that would solidify that your match is your half sister.

There can be some other oddnesses that I'm not addressing. But basically you're either a half sister or an aunt/niece to this woman.

The number of segments is not relevant here. Because of the closeness of the match, you expect it to be large. A closer match would have fewer segments (because of fewer breaks) and a more distant match would also have fewer segments, though it can vary. The number of segments here just tells you what you already know.

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