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Thread: Chromosome View in 23andMe Showing a Lack of Recombination Over the Centuries

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    Chromosome View in 23andMe Showing a Lack of Recombination Over the Centuries

    I noticed on 23andMe, in the Chromosome View in Ancestry Composition, that my Scandinavian, French and German, and Iberian all show up as really large segments, and and all the varying percentages that exist lying within a few large chunks. Being nearly all British in origin dating back to the early eighteenth century, I don't see how this can be accurate. I can understand British making up large chunks, and as nearly all my ancestry is British. However, I have no Scandinavian or Iberian ancestry dating back until at least the early 1700s, and even if I did have that ancestry dating back to the 1700s, they should appear now as small, fragmentary chunks. Do these large segments get passed down generation after generation for centuries without recombining? Or are these large segments really just big ol' overgeneralizations, such as the end points represent the said ethnicity, and everything in the middle is just connecting those two endpoints. I noticed what I describe above not only on my chromosome view, but in several relatives who are mixed American mutts too. I don't see how these big segments come down unrecombined for centuries for so many people.
    Last edited by can't_lurk_no_mo'; 11-01-2016 at 11:13 PM.

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    I have a few big sections of Scandinavian on 23andme as well. No known Scandinavian ancestors but I am half northern euro, British and German of more recent immigrants to the US. It may reflect parts of Germany or Britian that look more Scandinavian. Or maybe there is a unknown person of more recent Scandinavian descent in my ancestry. I get quite a few distant Swedish and Finnish matches on Gedmatch. One or two Norwegian. I don't show Finnish on 23andme, just minor Scandinavian.

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    Quote Originally Posted by can't_lurk_no_mo' View Post
    I noticed on 23andMe, in the Chromosome View in Ancestry Composition, that my Scandinavian, French and German, and Iberian all show up as really large segments, and and all the varying percentages that exist lying within a few large chunks. Being nearly all British in origin dating back to the early eighteenth century, I don't see how this can be accurate. I can understand British making up large chunks, and as nearly all my ancestry is British. However, I have no Scandinavian or Iberian ancestry dating back until at least the early 1700s, and even if I did have that ancestry dating back to the 1700s, they should appear now as small, fragmentary chunks. Do these large segments get passed down generation after generation for centuries without recombining? Or are these large segments really just big ol' overgeneralizations, such as the end points represent the said ethnicity, and everything in the middle is just connecting those two endpoints. I noticed what I describe above not only on my chromosome view, but in several relatives who are mixed American mutts too. I don't see how these big segments come down unrecombined for centuries for so many people.
    It is in part due to 23andme smoothing algorithm, where smaller segments are overwritten in favor of larger ones.

    Then we process those assignments further, "smoothing" them out. For instance, if you have a long run of assignments from population A, interrupted by an assignment to population B, this process can correct that B to an A.
    https://www.23andme.com/en-int/ances...osition_guide/

    Another explanation is that Scandinavian also includes Saxon ancestry, which is common in the British genepool, hence many smaller segments are shuffled around repeatedly within the genepool mimicking larger segments...

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