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Thread: Intermarriage

  1. #1
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    Intermarriage

    One pair of my father's Great-Grandparents were 1st Cousins.

    On a DNA Testing Site like Ancestry, if someone is related to you via the intermarried pairing, is it possible to share more DNA than what your actual relationship is? (specifically, sharing 258cM with someone who cannot be your 2nd cousin, and is most likely your 3rd)

    Also and somewhat related: does Ancestry overestimate the centimorgans shared between two people?

    I hope that made sense *haha*, and thank you for any assistance!
    AncestryDNA Genetic Communities
    Father: Saint Lawrence River French Settlers, Newfoundland English Settlers, Scotland, New York Settlers, Nova Scotia & Massachusetts Coast Settlers
    Mother: Southeastern New York Settlers
    Paper Trail: Irish: 35%, English: 28%, German: 19%, French: 9%, Scottish: 7%, Dutch: 3%
    AncestryDNA: England, Wales & Northwestern Europe: 70%, Ireland and Scotland: 30%

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     emc (02-06-2019)

  3. #2
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    I have the same question. All of my grandfather's grandparents were 1st cousins, so my great-grandparents were double second cousins, or something like that. I share 33cM with a Portuguese woman, and FTDNA says we're 3rd cousins to 5th-cousins, but we have done a lot of research and still haven't found our common ancestor. Sometimes I think these cousin marriages make we seem genetically closer than we really are.

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     kujira692 (02-09-2019)

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    Yes. That's kind of a minimal version of endogamy. I have a whole bunch of double cousins because Beauchamp and McElroy family had a couple children from one family married a couple from the other family. So those double cousins always look closer than they really are.
    Daughter's ggg grandparents(father to left, mother to right)
    Male: R-BY4242, R-Z344, ?, ?, E-V13, ?, E-L29, ?, R-Z20907, ?, ?, ?, R-L2, R-L20, ?, ?
    Female: ?, ?, ?, ?, ?, ?, ?, W6a, ?, N1b1, ?, B4a1a1, ?, ?, ?, H3h, ?, ?, ?, ?, ?, ?, ?, I3a, ?, ?, ?, ?, ?, ?, ?, K1c1g

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     kujira692 (02-09-2019)

  7. #4
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    I tried to get a genetics guy to work this out and it seems although on average if you reduce the number of ancestors say 8 gparents get reduced to 4 then you have effectively lost a generation but in the process the dna gets chopped around a bit more. So it cannot be predicted exactly. Total length in cM should look more like the effective number of ancestors but longest length is what the companies weight by and that will be probably shorter.
    What you have to consider (and you do not know) is how many generations close cousins have been marrying each other and what the total population size may be.
    That might dominate your question.
    I found a 15th cousin (common ancestor ~1550) with a 12cM in a sub-section of a population for me, but that does not mean 33cM for your cousin is 1650!
    Image “Westray wifie” replica of Neolithic figurine Hidden Content
    Out of 64 pre 1800 births 45% Cheshire, 1% Irish (or Scottish), 25% south Derbyshire, 13% Burton on Trent area (where 4 counties within 10 miles), 7% Shropshire, 1% Staffs, 8% Lancs. So far all British Isles despite what some testing companies say.

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     kujira692 (02-09-2019)

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