Can inter-related cousins be false matches?

Question

Is it possible to have false positive matches for multiple people all linked to each other and to me? Ancestry does not show how many centimorgans are shared and I have half a dozen cousins listed as 4^th to 6^th cousins and one as third cousins. Based on their trees, they all share the same line of family that I don’t know how I am related to. It could be a NPE, but is there any possibility that many inter-related matches could be all false matches?

Comments

This is far from definitive (hence a comment instead of an answer), but I have noticed many relatives from a particular line that usually show up as 4-6th cousins, but I'm pretty sure they're more distant (often showing up in FamilySearch as 10th cousins, too many steps for the general WikiTree relationship finder). Since they came from an endogamous culture (Seventh Day Baptists), I suspect they have multiple intertwined relationships to me, and those more distant but multiple relationships give more shared centiMorgans (and thus a closer predicted relationship).

It's also possible that I just haven't found that one closer relationship, too.

Answer 1

If they are all related and you are not, then the logical reason would be that  there may be an NPE in YOUR tree, not in theirs...

Have you uploaded your DNA to Gedmatch? Are any members of this other family matching you on Gedmatch? Gedmatch does show shared centimorgans...

Comments

Yes, I am on Gedmatch. I have asked a few of those cousins to register on Gedmatch but none has done it so far.

Thanks,

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It could also be that you are connected at that level through the other surname you share. Perhaps an ancestor you haven't found at this point.

Recently I was contacted by someone I've known, surname Burnett, and I was showing as a match at FT, also my Mom. Turns out we share that close connection through surname Martin, which Burnett had failed to mentioned,.... then he looked at my wikitree. We are 4th cousins. AHA!

Answer 2

Are you sure about not showing CMs? Ancestry should show you CMs for every match. Click on the little "i" next to the Confidence level. On the very distant matches I think it is possible to have a random false positive (there's one person who matches me at 9 Cms, but doesn't match my dad or my maternal grandparents), but I don't think that's very likely at the 4th and 3rd cousin level or with multiple people

Comments

Thanks for the tip. I had no idea the "i" shows the amount of centimorgans.

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Very good! They practically hide it, but it's one of the most useful features there!

They also seem to just about hide the "Shared Matches", which is a tab you can click on when you look on the details of a match, and which is the most important feature. Be aware that the Shared Matches list only includes matches down through the "4th cousins" level (which is down to 20cM). So you can tell if a "Distant Cousin" matches a so-called "4th Cousin", but not vice versa.

Opinions vary on EXACTLY how low you can go before worrying about false positive matches, but pretty much everybody agrees that at least 15cm is safe. That's the minimum value for a "Good Confidence" match, so for "Good Confidence" or BETTER matches you should be confident that they're all real matches. That takes you well into the "Distant Cousins" category, so ALL your "4th cousins" level matches, or better, should be legit matches.

Answer 3

It is possible that you are related but show no DNA match.  This happens most often with people related farther back than 3rd cousins.  

How does this happen?  Chalk it up to recombant DNA.  

I have a paper trail 4th cousin.  We are a lot alike too.   We did DNA and lo and behold no match shows for us.  Yet we both match others...   So what happened?   The way our DNA came down was diluted so that a match does not show between us but does show between us and others.  

Think of it this way...  

50% from each parent  (safe bet no false readings)

25% theoretically from each grandparent maybe...  DNA has started recombining so you might get 30% from Grandparent A and 20% from B and 45% from C and 15% from D.

Great Grandparents 8 DNA donors 12.5% theoretical.   Once again we have DNA recombining so remember in our above example the one at 15%?  It may be totally gone at this point.  So it appears like you do not have a match to that line.  But you are still related even if the DNA does not expressly say so.    

I have seen something very similar to this with one of my 4th cousins.  We DNA match to others... we both match to these others but not to each other. She is from my Dad's Mom's line.  When I look at my total DNA % I have 57% United Kingdom.  The only line that comes from there is my Dad's Dad's branch.  So it is obvious that I got more from his Dad'd Dad's branch than from his Dad's Mom's branch.  My Mom's side is 100% Germanic / Swiss.  So the likelihood of it coming from her side is rather small.

Answer 4

If it's a small match to all of them, then it's also possible that you are Identical by State (that is, by coincidence) with their shared ancestor, and they've all inherited the same bit of DNA.

Comments

Let me give you some specifics. One of them shows as 4th cousin and she shares a total of 34 centimorgans on 4 segments. Her uncle shows as third cousin and he has a total of 95 centimorgans on 6 segments. Do you think it could be coincidence? I read that having multiple segments is a sign of closeness more than the total of centimorgans shared. If true, I think coincidence is less likely. What do you think based on the specifics I gave you? This is important to me because it can be the start of finding a NPE in my tree. Thanks again.

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Harry, your "3rd cousin" is JUST over the threshold to get him in the "3rd cousins" category, which is 90cM. In my experience on AncestryDNA, 95cM is slightly lower than my lowest 2C (which is 102cM, with 7 segments); it's middle-of-the-road for a 2C1R, or it's a pretty strong 3C. But 2C, 2C1R, and 3C should be thought of as "categories" - 2C has the same cMs as a half-1C1R, or a 1C2R, etc, for example.

95cM is too big to be more distant, unless you're related to him in more than one way (which can be a very real possibility).

I've got a 3C with 36cM, on 4 segments, so if I had to bet I'd say the uncle is a 2C to one of your parents (2C1R to you), and his niece is a 3C to you. But, again, this is very inexact, and in endogamy comes into it "all bets are off".

If you happen to have a sibling who has also tested - or even a 1st cousin on that side - seeing what numbers THEY have for these matches can also help.

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That looks more like inheritance to me. Taking the average length by dividing total by number of segments, indicates the largest segment must be at least that size. The fourth cousin has an average of 8.5. Her uncle 15.8 which is about double which you would expect.  The uncle must have a largest segment of at least 15.8 which is a solid match to my mind.

I suggest you look for which branch of your tree they are related to. Press the View Match button next to the 3rd cousin, the closest match. Then on the next screen press the shared matches button and look for closer relatives like 2nd or 1st cousins you do recognise. One of the two shared ancestors (husband and wife) between you and that recognised cousin is likely a descendent of the common ancestor of you and the mystery third cousin.

Tim

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I forgot to add that I haven't found the number of segments to be useful - it just doesn't seem to reliably tell you anything that the centimorgans doesn't already tell you. Usually, it's about 15cM per segment, like the uncle here, but it can vary widely (like with the niece here).

Answer 5

Harry, I'm giving you a gold star! The impression I get is that few people get as far as you have in analyzing their matches. Heck - half the people who match me on AncestryDNA have only tested within the past year! Practically everybody is new at this, so the first thing you have to be aware of is that you might be "ahead of the curve".

But take it from one of the few who has analyzed the whiz out of his own AncestryDNA matches. They're definitely NOT false matches, and while an NPE is possible, it wouldn't be my best guess.

It sounds more like a couple of match groupings I have, except smaller, and more closely-related. One of these groups has over 55 matches, who all match each other in some way. The highest one is 65cM, and is a known 3C1R (actually she's related 2 ways, so also a 4C1R). Between the handful of known relations, and the fact that a bunch of them have a Heckathorn in their tree- even though they don't go back far enough to make the connection - it's pretty clear that they're related through my gt-gt-gt grandma who was a Heckathorn, and came from a huge family. A few of us are on GEDmatch, and match on a common segment.

One of the known relations is a half-6C, so if he's not also related some other way, I know EXACTLY where that segment came from - my gt-gt-gt-gt-gt grandpa Heckathorn, who came over on a boat from Alsace in 1750! That might be kind of a "big IF", but it would really just make sense.

The thing is, my segment there is about 42cM long. On the one hand, that's unusual to have from that far back. On the other hand, there are so many ancestors that far back, that you're likely to have some of these! They call these "sticky segments" - segments that have managed to "stick together", not being chopped down by recombination, over many generations.

I have another group that's just as big, and all I can figure is that they must connect up within the unknown ancestry of my gt-gt-gt grandpa Standley. I've found people in those matches who are VERY distant cousins to each other, going back the a couple in the late 1700s. The surnames involved existed in the same place as my Standleys lived in the 1700s.

So what you have is not a PROBLEM, but rather an exciting discovery - one that might help you break through a "brick wall" someday! Whether it's just a NPE instead, depends on your specific circumstances.

Anyway, be aware that segments can persist for many generations - by sheer chance - and you could be looking at some crazy-distant cousins, mixed right in with your not-as-distant cousins!

Answer 6

Hi Harry, your matches may not be on GEDmatch but that does not mean you cannot compare each and every one of them to you and to each other. On Ancestry use the Shared Matches button to do that. Note down the matches that come up for each of those matches and how they match you and create a diagram of that information by drawing connecting lines between each of the matches (remember to include yourself in the diagram). You may see some that match everyone and others that match only one or two of the others. You will see how you may be connected to them. Use their family trees to work out their relationship to each other. And as you do have their family trees you will be able to use the Map and Locations button to see what geographical area they are in. Do any of your ancestors come from that same or similar area? That is where I would start looking for your connection to them. Remember that not all connections are down the direct line, your diagram will tell you whether that is the case. Also it is possible to share a match with others (in common with matches) but not share DNA with the actual match directly. Good luck with your research and I hope it goes well and fruitful for you.

Comments

I have used the shared matches to find out which branch of family they have in common. One of them has a tree going back to the 1500. I am almost positive about he MRCA with those cousins. If that girl is a 4th cousin and her uncle is a third for sure, I can see the name of the common ancestor. He was a Frenchman who came from France to the country I was born. He was born in 1800. Based of a membership I have with a website that list family trees from my country, I can see the whole descendance of that MRCA. I wanted to be sure that my case was not a coincidence. Now I am believing it is not. Those people are all related to me and to each other, they come from the same country as myself, and some of them share from 30 to 95 centimorgans on multipe segments with me. The more I think it over, the more I think I am into something. Now, what would be the method to go down from that MRCA to who in that descendance is related to me? That is the question now.