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  • Double cousins

    Some questions on double cousins here. I understand that the (respective) offspring of a brother and sister ("male A" and "female B") who marry a sister and brother themselves ("female C" and "male D") are considered “double” cousins. But what about subsequent generations? And what if subsequent cousin generations are once removed? I‘m trying to understand that “official” relationship/designation.

    I understand that, genetically speaking, 1st cousins would normally share about 12.5% of their DNA. However, double cousins instead share about 25% of their DNA – the same amount that a person would with a grandparent, half-sibling or aunt or uncle. That then filters down to each generation outside the expected norms (loosely speaking 50%, 25%, 12.5%, 6.25%, 3.125%, 1.5625%, .78125%, .390925%, etc.)

    Here’s my scenario:

    My 2x great-grandfather (“Samuel”) and 2x great-grandmother (“Catherine”) are also the 2x great-uncle and 2x great-aunt of my cousin (“Cousin C”). Similarly, Cousin C’s great-grandfather (“Alexander”) and great-grandmother (“Mary”) are also my 3x great-uncle and 3x great-aunt. As you can see, the relationships are one generation between me and Cousin C, hence, the “once-removed” designation.

    What are we to each other? From my perspective Cousin C would appear to be a 3C1R. Is that correct? But does the “double” cousin aspect alter that conclusion?

    Color me confused!

  • #2
    I believe technically the double will filter down, Double 3C1R, due to fact you each share two of the same common ancestor couples
    4 of your 3x Great Grandparents match with 4 of "Cousin C"'s 2x Great Grandparents

    cM wise I would assume DNA share would be more in the 2C1R range

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    • #3
      Hi, prairielad. Thanks for responding. As an FYI according to FTDNA, me and Cousin C share 75 cMs, with a long segment of 30 cMs. Alternatively, Gedmatch shows us at 51 total cMs with a long of 35 cMs. But, yes, I do understand that my 3x great-grandparents are Cousin C's 2x great grandparents.
      Last edited by mjclayton31; 4th November 2018, 12:24 AM.

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      • #4
        Originally posted by mjclayton31 View Post
        Some questions on double cousins here. I understand that the (respective) offspring of a brother and sister ("male A" and "female B") who marry a sister and brother themselves ("female C" and "male D") are considered “double” cousins. But what about subsequent generations? And what if subsequent cousin generations are once removed? I‘m trying to understand that “official” relationship/designation.

        I understand that, genetically speaking, 1st cousins would normally share about 12.5% of their DNA. However, double cousins instead share about 25% of their DNA – the same amount that a person would with a grandparent, half-sibling or aunt or uncle. That then filters down to each generation outside the expected norms (loosely speaking 50%, 25%, 12.5%, 6.25%, 3.125%, 1.5625%, .78125%, .390925%, etc.)

        Here’s my scenario:

        My 2x great-grandfather (“Samuel”) and 2x great-grandmother (“Catherine”) are also the 2x great-uncle and 2x great-aunt of my cousin (“Cousin C”). Similarly, Cousin C’s great-grandfather (“Alexander”) and great-grandmother (“Mary”) are also my 3x great-uncle and 3x great-aunt. As you can see, the relationships are one generation between me and Cousin C, hence, the “once-removed” designation.

        What are we to each other? From my perspective Cousin C would appear to be a 3C1R. Is that correct? But does the “double” cousin aspect alter that conclusion?

        Color me confused!
        I'm published as a traditional genealogist, and I don't consider myself an expert in genetic genealogy. But I have seen marriages resulting in double first cousins in relatively recent generations of my own family. They occur in endogamous societies and families - a lot. Undertaking an understanding of history might broaden your perspective of why this occurrence was so common.

        The ultimate and confusing puzzlement occurs when a very large percentage of a village of ancestry, (sometimes half or so) over many generations marries their distant cousins - all of whom have the same surname. To make matters even more confusing, some have the same given names, as well as the same surnames - and sometimes their kids have the same names, but are born in different families, sometimes having the same year of birth as their cousins. You ain't seen nothin' yet... :-)

        So you think you've got it tough? And you think you are confused? The primary key to unraveling puzzles such as this is through doing record searches. Genetic genealogy is a tool that can provide answers for some puzzles. But it's not a magic bullet. It can't and never will solve every puzzle. It will tell you that you are related to someone. But it can't tell you specifically how you are related.

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