Low Ancestry cM matches but paper trail suggests correct

[smudwhisk]

According to Ancestry's matches, I only have one 3rd cousin who has tested (101cM across 5 segments) but I do have another who appears as a 4-6th cousin sharing only 23.8cM across 1 segment. The paper trail does confirm she is my 3rd cousin, but the amount shared is much lower than the other known 3rd cousin. Interestingly we also have a shared match with someone else with whom I share 65cM over 4 segments.  He doesn't have a tree on Ancestry sadly but his user name suggests he's a relative of the 3rd cousin (the username includes the surname of my 2xGGUncle's wife, brother of my GGrandmother and Great Grandfather of the other match).  65cM on DNA Painter's Shared CM Project has more likelihood of half cousins than full which makes me wonder.  Our 2xGGrandfather ended up in a mental asylum not long after 2xGGUncle's birth, and his wife did go on to have an illegitimate child some years after while he was in the asylum.  Makes you wonder whether he wasn't 2xGGfther's son!  But I'm just speculating there although it would explain the low match level.    I've found similar anomalies with 4th cousins, many of which are 15-25cM only yet confirmed by the paper trail. I don't seem to have inherited a lot of DNA across those lines.

However, one very low match I'm rather interested in but wonder if it could just be an anomaly in spite of the likely paper trail.  I've one 5-8th cousin match who shares 6.1cM across 1 segment. Very low but her tree suggests 7-8th cousin on a line that I'm pretty sure is correct but there has always been a bit of a doubt because my last direct female ancestor on that line never used her middle name after she married (which would have confirmed her identity beyond any doubt and she died before 1851). However, balance of probabilities suggests that the research is correct but should I read too much into a link with someone at that level who has the family on their tree?

Out of 247 4th cousin or nearer matches, the vast majority are less than 25cM. Those with a bit more don't have trees and I can't quite work out where they link in. Bit frustrating but early days. I have managed to confirm a few pieces of research which has been helpful.

[melba_schmelba]

Yes it is possible. Third cousin is great great grandparents in common, and it is not completely impossible to lose all of a great great grandparent's DNA, so to inherit an abnormally low amount is quite possible.

[smudwhisk]

Thanks Melba, yeah I meant our great grandparents were sibling so share 2xGGparents.  I've messaged the person with the larger match to see if he responds and confirms where he fits in out of curiosity.  Whether he replies remains to be seen, he has logged in this week.

Any thoughts on the 6.1cM match which the paper trail would suggest is correct?

[melba_schmelba]

Thanks Melba, yeah I meant our great grandparents were sibling so share 2xGGparents.  I've messaged the person with the larger match to see if he responds and confirms where he fits in out of curiosity.  Whether he replies remains to be seen, he has logged in this week.

Any thoughts on the 6.1cM match which the paper trail would suggest is correct?

I had one at that level that had a tree match to a common ancestor born in 1700. It's possible that we do inherit DNA from that ancestor, but unless, you and that other person have their ancestry traced in full, back that far, and, perhaps you can find other people that share the DNA and have that tree link (on Ancestry or GEDMATCH), you can't be sure the DNA wasn't from another line.

[smudwhisk]

The person's tree only goes back on that line to a birth in the early 1820s, mine however goes further back and comes forward in detail, hence the reason I know its that line.  The mutual ancestors would be my 6xGGparents born c1730, not sure as haven't checked exactly which cousin level she would be but it is at least 7th cousins.  Don't match anyone else so far through that line.  Usual issue, not many people seem to have tested on my known lines and the links are generally in the 1700s.  I am happy my research is correct so the 6.1cM could well be correct.  Just have to hope that someone else tests, unless I can persuade another RCer descended from the same line to test but it will depend on how much they inherit from that line as to whether we'd match.   They are closer to this person in the descendant line than I am so may match them but not necessarily me.

I do seem to have rather a lot of low level matches, some of which can definitely be confirmed by the paper trail.  A third cousin once removed sharing 16cM over two segments that Ancestry has listed in the 5-8th cousin range.  I know the paper trail is correct and we both match what is my 2nd cousin twice removed (my grandmother's second cousin but much younger) which is her 1st cousin once removed.  I only share 33cM over two segments with the 2nd cousin twice removed.

I just wish Ancestry would show 5-8th cousin shared matches with all and not just the 4-6th cousin shared matches when you look at someone linked at the 5-8th cousin level.  It could quite likely show some more links.

[melba_schmelba]

I just wish Ancestry would show 5-8th cousin shared matches with all and not just the 4-6th cousin shared matches when you look at someone linked at the 5-8th cousin level.  It could quite likely show some more links.

Yes this is annoying, as we discussed in the other thread . That is where GEDMATCH can be helpful as it will show shared matches down to, I think, 3 centimorgans, although it certainly doesn't advise going that low. For one example, I tried recently, even limiting myself to 10, I got quite a few more shared matches than I did on Ancestry.

[smudwhisk]

I'm thinking about uploading to Gedmatch, just not got round to it and I believe some features you now have to pay for, unless I'm misreading some posts.  Its early days anyway as I only got the results on Thursday, just finding it quite fascinating and frustrating at the same time.

[smudwhisk]

Just wondering what the likelihood would be of half 6th cousins inheriting 9.2cM over 1 segment?  Just wonder if it could be too high, albeit that I seem to have a lot of differing levels for closer cousins that can be confirmed by research compared to Ancestry's estimates of relationship and perhaps I've inherited more on other lines.  I've found a connection that could confirm the parentage of my 5x GGrandfather which circumstantial evidences suggests could well be true.

[Alexander.]

I have a third cousin who shares only 8 cM with me, another one with 35 cM, and another third cousin with 198 cM. It just shows the randomness of how DNA is inherited and how much variation there can be even within just a few generations.

With small matches, especially under ~50 cM, there becomes a huge number of possible relationships, and it's really down to tracking the paper trail.

I like the DNA Painter tool to assess probabilities of match relationships, see https://dnapainter.com/tools/sharedcmv4

Alexander.