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My father's half sister just posted her AncestryDNA results.

I did not come up as a match, nor did my sister.

Does this mean that my father's dad is not really his dad, or could there be another explanation?

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It absolutely means that your half aunt is not biologically your half aunt or another close relative.

There are 4 possibilities:

  1. Your father's dad is not his biological dad.
  2. Your aunt's dad is not her biological dad.
  3. Your dad is not the biological dad of either you or your sister.
  4. There is some sort of error at Ancestry.com. If you're checking your account and your sister's account, maybe your aunt's results aren't fully up yet. But if you're looking through your aunt's account, that isn't the answer. Outright errors are rare but you'll want to rule them out. Are there any other expected relatives she does or doesn't match? Any that you and your sister do or don't match?

(I'm assuming from your question that you already know that your dad and his half sister have different mothers and are assumed to have the same father.)

  • What's the minimum number of cMs that Ancestry uses to judge whether two kits are a match or not? – Jan Murphy Feb 6 at 18:31
  • @JanMurphy 6 cM. They also use Timbor, a program that removes some segments from consideration because they consider them to be too common (more complex than that). But it is completely impossible they'd miss a half-aunt match. Could never happen. isogg.org/wiki/Autosomal_DNA_match_thresholds – Cyn Feb 6 at 18:50
  • I know about Timber and how Ancestry uses it to remove pile-ups. My main concern here is that none of our answers so far have any content in them that would further a newcomer's understanding of the science. We're just expecting them to take our word for it instead of explaining why. But I asked in my comment because I don't have a kit at Ancestry and I am depending on others to report what Ancestry shows the user. – Jan Murphy Feb 6 at 19:02
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    @JanMurphy As you know, I generally give cM ranges for relevant relationships. I think it's pretty obvious here that zero is not anywhere near the range needed for half-aunt. It is unnecessary to give that range. And, in fact, it complicates and clutters the answer. I answered the OP's question. I did not pile on lots of technical information that might or might not be of interest. The OP is welcome to ask followup questions. Right now, we need more information to even guess where s/he wants to go with this. – Cyn Feb 6 at 19:05
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    @JanMurphy We know it's so low that it might as well be zero. This is a situation where any number low enough not to show as a match can be rounded down to zero with no loss of information. I see that you are criticizing my answer, but I have no idea why. It is not just unnecessary but outright confusion to add in numbers like you are suggesting. But if you feel my and PolyGeo's answers were inadequate, I'd love to see yours added here. – Cyn Feb 6 at 19:17
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If your father's "half sister" is expected to share a father with him then one alternative may be that your father has his expected father but his "half sister" does not.

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From your description, your paternal grandfather had two wives. One is the mother of your father and the other is the mother of your aunt. @cyn's point

  1. Your aunt's dad is not her biological dad

is the most likely scenario. I would look for a previous marriage for your aunt's mother or a birth record for your aunt in her mother's previous married name, or her mother's maiden name.

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