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Do Double relationships affect DNA matching?

Do Double relationships affect DNA matching?

Posted: 5 Aug 2012 12:16AM GMT
Classification: Query
Edited: 5 Aug 2012 1:59PM GMT
Among my ancestors were three families that began in early Maryland and then migrated south and west more or less together for several generations. There were multiple marriages between those three families over the course of 3 to 4 generations, including some marriages of cousins. Is it just coincidence that the bulk of the matches that I have had returned (those I can show on paper) seem to match up those families or did the fact that there were several double relationships increase the odds of matches being made between their descendants? Or maybe it is only because those families have been pretty well documented on paper and a lot of folks have access to that documentation? It just seemed odd to me that the ONLY matches that have been returned to me that I can match on paper involve those rather remote branches of my family....and are showing as only moderatly confident....None of the 95% confident matches have seemed to be connected.

Re: Do Double relationships affect DNA matching?

Posted: 5 Aug 2012 6:13AM GMT
Classification: Query
That's probably due to both intermarriage and well-documented families. If you or another tester are descended from the same ancestors on more than one line, you're likely to inherit more than the expected average DNA contribution from those ancestors. In the case of multiple siblings from one family marrying multiple siblings from another, the effect should be similar.

Also, when a good paper trail exists for a family, such as a quality published genealogy, lots of the descendants are likely to trace those ancestral lines back to the progenitors.

Re: Do Double relationships affect DNA matching?

Posted: 20 Sep 2013 5:53PM GMT
Classification: Query
I have a somewhat different (multi-part) question on the same topic:

IF you have double-cousin relationships in a family tree, and since this theoretically suggests more DNA locus matches than for a single cousin relationship, will AncestryDNA estimate the double cousin to be closer to me in generations (e.g. a "3rd or 4th cousin") rather than a proper placement in a more distant generation (e.g. "5th or 6th cousin")? That is, does the double-cousin linkage "fool" or confound the generational placement estimated by AncestryDNA?

In addition, does double-cousin linkage artificially increase the confidence level (e.g. "95%") of AncestryDNAs for a suggested match?

Re: Do Double relationships affect DNA matching?

Posted: 21 Sep 2013 5:10PM GMT
Classification: Query
@Chuck,
exactly my question as well. We have a match that looks like a 3rd to 6th cousin but the 2 MRCAs are further back, around 8/9 generations

Re: Do Double relationships affect DNA matching?

Posted: 21 Sep 2013 6:01PM GMT
Classification: Query
Edited: 21 Sep 2013 6:09PM GMT
They will very likely show up as closer matches. The amount of autosomal DNA inherited from ancestors beyond your parents is entirely random, so that there is no way to know for certain if an unexpected close match is due to a cousin marriage without the paper trail.

Re: Do Double relationships affect DNA matching?

Posted: 22 Sep 2013 5:00PM GMT
Classification: Query
Yes. The predicted relationship will likely be closer than it is in actuality. For instance, i have a predicted 3rd cousin match to a lady who is actually a 5th cousin. This is due to 2 different lines that we share, and in her case the descendants of these two lines intermarried and produced her ancestor. Once we shared our paper, the true relationship was quickly established, but without our documentation, we would have been looking for a closer connection.

Re: Do Double relationships affect DNA matching?

Posted: 27 Sep 2013 9:32PM GMT
Classification: Query
In our family we believe that my great grandparents on my paternal grandmother's side were cousins.

I have a second cousin on FTDNA from this line . Her maternal grandfather and my paternal grandmother
were siblings. we share over 309cM's

FTDNA gave a relationship range of first-third cousin and suggested a second cousin relationship. 309 is high for second cousin, so I' m not sure how they arrived at that.


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