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I have spent hours today looking at my 23andme DNA matches, and while I have not checked every single one, it seems to me that ALL of them descend from ancestors of my paternal grandmother. I am 100% sure of about 3 of them, and so far every person in the list matches someone (that matches someone..etc..) that shares DNA with these 3. I am not aware of any pedigree collapse. I know the birthplaces of all but one of my 32 3xgreat-grandparents. All are very distinct geographically, between grandparents, except for a handful born in "Ireland".

I already know that these particular ancestors had many children etc, and that in my far-from-complete Visual Phasing exercise I have a particularly low amount of DNA from my father's father. However I am not satisfied that this explains why all 1500 23andme matches are linked to one grandparent.

This phenomenon does not occur on other DNA sites holding my DNA (but they have many more matches).

None of them have only one tiny segment, lowest match of the 1500 has one seg of 16cM.

Could it be some kind of mistake on 23andme's part?

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    We need a lot more information to answer your question. How many relatives whose relationship you know are among your matches? How much do they share and how is each related? What's your closest unknown match? What technique are you using to determine they are all on one grandparent's side? Leeds? Does 23andMe produce a tree for you? – lkessler Feb 9 at 13:52
  • I have deduced with certainty that three or more matches are definitely on this grandmother's side (via collaboration on family trees). These are not people I knew before taking the test. They are 3C1R, 4C and 4C2R. My closest unknown match is estimated to be 3C, with 78cM. My technique is simply to note the matches in common with me and one chosen match. No matter who the chosen match is (I've tried over a hundred), the same names keep coming up. Sorry what is "Leeds"? Yes 23andme produces a tree - and does propose that matches hang off different grandparents. Thanks!! – Victoria T Feb 9 at 18:31
  • Please use the edit button beneath your question to revise it with any requested clarifications. – PolyGeo Feb 10 at 6:17
  • Doesn't help, but I have essentially the same problem. All of my 23andMe matches seem to connected in the same way. The shared matches tool isn't very good in my experience because pretty much all of my matches show up as shared matches of one another! – Charlie Feb 11 at 0:36

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