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My first cousin twice removed married my great-great aunt, so what is my first cousin twice removed’s four-times-great-grandfather to me?

Do we share common ancestors?

closed as unclear what you're asking by Janus Bahs Jacquet, Jim, choster, Hellion, Robusto Feb 18 at 1:03

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    Please explain the circumstances in which you have a first cousin twice removed. Are your grandparents his/her great great grandparents? Or are your great great grandparents his/her grandparents? Is the person concerned older or younger than you? – WS2 Feb 17 at 18:28
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    What is the “4” in your question? – Jim Feb 17 at 20:20
  • Obviously if you have a blood relationship with each person in question then you share a common ancestor. If your relationship to either of them is only “by marriage” then there is no common ancestor. – Jim Feb 17 at 20:24
  • @Jim Because cousins share only half their grandparents, then if we assume no inbreeding there’s only a 50% chance of those two individuals having a common ancestor. – tchrist Feb 17 at 23:17
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    @tchrist If the person described is, without qualification, a "cousin", no matter how many times "removed", they must have a common ancestor. "Removal" simply means son/daughter, grandson/granddaughter of. – WS2 Feb 18 at 23:31
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This is a matter not of English but of math, and not merely arithmetic but deductive logic. The simple answer is that because cousins share only half their grandparents, then if we assume no inbreeding there’s only a fifty percent chance of those two stated individuals having an ancestor in common.

Here’s how to work that out.

Your first cousin twice removed is the first cousin of one of your grandparents. Let’s call him C.

Your great-great-aunt, sometimes called your great-grandaunt, is the sister of one of your great-grandparents, which makes her one of your grandparent’s aunts. Let’s call her A.

In other words, when C married A, some grandparent’s cousin married some grandparent’s aunt. Those two people may have no common ancestor because we don’t know that the two grandparents just mentioned are the same person (GX vs GY perhaps), or even both on the same side of the family. Although you (Y) are of course related to both A and C and thus share a common ancestor with each (YAP and YCP), it may well be that those two common ancestors YAP and YCP are unrelated to each other. This is how it works in most families.

You’ve asked whether C’s great-great-great-great-grandfather (G5F) is related to you. The answer is that we cannot know this, for such is the nature of cousins. It’s because C is your grandparent’s cousin, and cousins normally share only two of their four grandparents in common (barring any inbreeding). That means that they presumably both have another pair of grandparents not shared in common between them.

So that’s all that counts: half are and half aren’t. Even though a person has two-raised-to-the-fifth-power great-great-great-great-grandfathers, making thirty-two of them, because cousins share only half their grandparents, they also share only half their great-great-great-great-grandparents.

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