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Thread: Cousin couples

  1. #1
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    Cousin couples

    I was doing my tree on Ancestry.com when I accidentally discovered the parents of my great-grandfather (my mother's maternal grandfather) were first cousins. Mr. Robinson (father's father) and Mr. Forbush (mother's father) both married Lewis sisters. Am I inbred? How inbred would my great-grandfather be? Keep in mind my great grandfather was a very hardy man; he sired ten children, all daughters, he lived to be 85 and remained lucid and active until shortly before his death in early 1993.

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  3. #2
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    You're not inbred, maybe if several of your great great grandparents were cousin marriages from the same population who themselves had cousin spouses in their recent history too. Then your family tree would start to collapse back on itself.

    It's pretty common but just not talked about very much because people are embarrassed about it.
    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: ?, ?, H3g, ?, ?, ?, ?, W6a, ?, N1b1, ?, B4a1a1, ?, ?, ?, H3h, ?, ?, ?, ?, ?, ?, ?, I3a, ?, ?, ?, ?, ?, ?, ?, K1c1g

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    Quote Originally Posted by Osiris View Post
    You're not inbred, maybe if several of your great great grandparents were cousin marriages from the same population who themselves had cousin spouses in their recent history too. Then your family tree would start to collapse back on itself.

    It's pretty common but just not talked about very much because people are embarrassed about it.
    I have a fair amount of pedigree collapse in my maternal grandfather's tree through multiple ancestral lines that sort of weave into each other at various points. Even with all that there are no signs of inbreeding in my family. Agreed that OP is not inbred.

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  7. #4
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    I agree, you'd need a lot of collapse and for it all to happen rather suddenly to be inbred. And once they start breeding with the population at large it's lost very quick.
    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: ?, ?, H3g, ?, ?, ?, ?, W6a, ?, N1b1, ?, B4a1a1, ?, ?, ?, H3h, ?, ?, ?, ?, ?, ?, ?, I3a, ?, ?, ?, ?, ?, ?, ?, K1c1g

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    Found a few in my family tree as well. I don't think there was a stigma about it until relatively recently in history.

    By the way, even Charles Darwin married his first cousin, and had 10 children with her.

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  11. #6
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    How about this for a measure.

    The sum from 1 to n where n is the number of generations from you forward where

    ((Unique Ancestors in generation n)/(2n))

    So at 8 generations I'm
    (1/1+2/2+4/4+8/8+16/16+32/32+62/64+122/128)/8 which puts me at 0.990

    You could run it out further but it's hard to keep track. I bet most people are between 0.9 and 1.0. My grandmother came from a small village in Slovakia and cousins and 2nd cousin marriages galore in the 18th and 19th Century.

    Or is there already a scientific measure for the degree of pedigree collapse a person has.
    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: ?, ?, H3g, ?, ?, ?, ?, W6a, ?, N1b1, ?, B4a1a1, ?, ?, ?, H3h, ?, ?, ?, ?, ?, ?, ?, I3a, ?, ?, ?, ?, ?, ?, ?, K1c1g

  12. #7
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    It's very very common. You could find a ton more and it'd be nothing to worry about.

    @Osiris " coefficient of inbreeding " is what i've seen used for this.

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  14. #8
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    Quote Originally Posted by xenus View Post
    It's very very common. You could find a ton more and it'd be nothing to worry about.

    @Osiris " coefficient of inbreeding " is what i've seen used for this.
    This wiki gives more info and it is defined the other way around, so a higher figure is more inbred. http://www.genetic-genealogy.co.uk/Toc115570144.html
    There has been considerable study of this effect see this for example https://www.sciencedirect.com/scienc...10863017300174 and a single first cousin marriage does not count as inbred, many generations of the same does. In some countries it is significant.
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    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.

  15. #9
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    Interesting... My father and his first wife are first cousins so that makes their children my half siblings but also 1st cousin 1 removed right?
    3/4 European, 1/4 Mauritian Creole. Genealogy enthusiast and Wow nerd.

  16. #10
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    Here's a piece on the history of laws against first cousin marriage in the US, that people might find interesting: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605922/

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