Bottom Line Up Front [BLUF]: Yes, but caution.
Here is the original statement that prompted this blog post:
The chance that three fourth cousins will all share the same matching segment is practically zero.
A bold statement – repeated several times – that has implications for Triangulated Groups. It appears this was part of the education material provided by AncestryDNA for their DNA Circles feature [Hat Tip to Debbie Kennett – the material is no longer online].
This means you and two 4C Matches sharing the same matching segment [all three of you descending from 3 different children of the Common Ancestor].
Mitigating factors:
1. Shared DNA segments in a Triangulated Group (TG) are rarely “the *same* matching segment”. We are almost always talking about overlapping segments of different sizes. So that gives some wiggle room. Maybe the odds are just small (not practically zero) with a group of different sized segments in a TG.
2. As Debbie pointed out to me, these were simulations by Ancestry, using “perfect” data. In genetic genealogy our data is usually somewhat messier than simulated data, so there is even more wiggle room. Maybe the odds are on the low end…
3. Another factor is that the data has grown substantially since the simulations were done for the Circles feature. The information has been removed.
The bottom line for me becomes: If you find 4C Matches in one TG from more than two other children of the Common Ancestor, take a closer look at it. It is possible, but there may be other factors at play.
Segment-ology CONCEPT – For Matches forming a TG (overlapping segments in a range), the odds decrease with each generation going back and with each additional child of the Common Ancestor. Take a critical look within TGs beyond 3C Matches spread over more than 2 other children. The odds are very small with Matches from 3 other children (total of 4 children). This is not a “rule”.
Important Note: This does not mean that we cannot have DNA Matches from 4 or more children. We can! Instead of a double negative let me say: We can have 4C Matches from more than 3 other children of the Common Ancestor – we can have 7C Matches from 5 other children of the CA. It just means that there is more than one segment (TG) involved. Over the different children, we should expect to see several TGs. We can have over a hundred Matches in a TG going back to 7XG grandparents, for example. We just need to carefully screen for the number of children per TG.
Takeaway: It’s hard to have a hard “rule” on this subject. However, it makes sense to pay attention to our data. The further back we go (in generations), the more constrained our options become.
I’m inviting discussion on this Segment-ology CONCEPT, and on your experience with TGs and numbers of Common Ancestor children.
[08E] Segment-ology: Can Three Fourth Cousins Share the Same Segment? By Jim Bartlett 20230812
segment-ology 
Sorry for the somewhat late question, and also slightly off topic, but anyway –
It occurs to me that if the chances of three or more fourth (or fifth etc ?) cousins sharing the same segment are very small then if I have matches who trace back to three or more children of a common ancestor then the chances of those matches being IBC is similarly very small. ie three siblings proves(ish) the connection.
Am I correct in this assumption ?
This would be mostly in reference to Ancestry matches with small cM matches which I can’t triangulate and which don’t have Shared Matches (way below the limit)
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Steve, No. There is a difference between Match segments in a Triangulated Group; and Matches, but not sharing segments in a TG. It is entirely possible for us to be cousins with Matches who descend from all the children of a Common Ancestor – it’s just that they will share different segments with us. In other words we will have several TGs from that CA. I have 372 TGs over my whole DNA. This mean that there are roughly 3 TGs for each of my 128 5xG grandparents; approximately 186 TG pass through each parent. Hope this helps, Jim
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Thanks for the reply Jim. I see what you mean. Still struggling to get my head round the logic of all this. Steve
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Jim ciao che vuol dire analisi DNA privato perché c’è una corrispondenza che ho visto nel loro albero che con me corrisponde la loro figlia mentre i suoi genitori nel albero hanno testato ma e scritto analisi del DNA privata.per vedere se uno tra il padre e la madre può corrispondere che vuol dire?
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Ciao jim volevo dirti una altra cosa di quel gruppo perché loro sono più vicini di parentele ho visto due corrispondenze di quel gruppo che ti parlo sempre e loro hanno una parentela più vicina e poi ci sono io e un po’ strano non credo che sia ha caso.tu che dici?
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Kevin, I say you need more Matches who Triangulate on the same segment(s). It’s impossible to look at many of our Matches in isolation and draw a firm conclusion. We need more input. Jim
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Ciao jim ho avuto altre corrispondenze tra stranieri e italiani ma non sono collegati ha quel gruppo quel gruppo e misterioso se e vero.ce una bambina ha testato i suoi genitori ma e privato.
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The only example I have been able to find so far in my own results is one TG that is shared between 5th cousins via three different siblings. The paper genealogy is solid. I am 4th-great grandson via sibling #1, Match B (36 cM) is 4th-great grandson via sibling #2, Match C (27 cM) is 4th-great granddaughter via sibling #3. Match B’s mother (my 4th cousin 1x removed) has also tested and she shares her son’s segment. Match B’s great aunt (aunt of Match B’s mother, and thus 2nd-great-granddaughter of the ancestral couple and my 3rd cousin 2x removed) has also tested and has a 35 cM segment that triangulates. Match C’s mother (my 4th cousin 1x removed) has also tested and shares her daughter’s segment. (All of these tests are on GEDmatch except that for the mother of Match B, who is on MyHeritage; the MyH Chromosome Browser shows a TG.) So, this seems to be an example fairly far out on the long tail of the curve.
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I found this fascinating and relevant to my work with a group of over 2000 descendants/researchers from a limited geographic area of SW Kerry Ireland. I compare multiple descendants at the third, fourth and fifth cousin level relying primarily on Gedmatch. I looked to see if I could find more than three separate lines in one triangulation, and I found an 18 cm possibility. The four matches are a subset of the 25 tracked descendants on Gedmatch across 5 children of a couple born around 1750. Three are 5th cousin’s and 1 is a 4C1R to the others. The complicating factor is that this family is from a small, relatively isolated area; the spouses of two of the children are also siblings so those lines are double cousins. However, a descendant on a third child’s great grandmother b. 1871 emigrated to Australia and has no other identified family from Ireland on other lines so that likely limits her connections. I also found it of interest that if you look at the match from the perspective of the 4C1R he appears to have inherited a total span of at least 55 cm from this couple: the 18 cm is in the middle of triangulations with other descendants of this couple which includes a 40 cm span with the Australia cousin. I’d love any feedback or suggestions. I have pictures which make this example clearer but realized I couldn’t attach.
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Deborah,
Thank you for this research and feedback. If I understand this, out of 2000 people (in a limited area), you found one instance of four 5C in one TG… This would then indicate the probability is indeed very small, and that it is not zero. It may be that there are other factors, but I’m inclined to take this at face value – a reminder that distribution curves have tails. Jim
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This was by no means a full comparison of our data: I wish I had a way to do that. As we look at the various family’s I run automated McGuire charts against a large tree database of combined research and then look at the comparisons of descendant’s segments in a file of matches from – at this point – around 430 Gedmatch kits.
This triangulation was the first one I found as I started checking families where I had 1) solid tree info on four or more separate children, 2) a fair amount of detail on many of the descendants’ DNA – chromosome maps etc., and 3) triangulation was of a reasonable size in cM. The fact that I found one in a couple hours searching tells me there may be more, but also not a lot. I wish I’d been aware of and tracked this as I looked at various families and their descendant’s matches. If you have any suggestions on how we can get more information from our data I’d be very interested. Because of the spotty nature of Irish records before around 1865 a lot of what we are doing is trying to confirm ancestors at this distance back. I’m very interested in any other metrics for the 3rd/4th/5th cousin level matches.Thanks, Debby
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Deborah, I think we are in agreement – it’s not impossible, but it’s not common either. And I’d add that at the 5C distance, there may be other explanations. But in the end, with more and more data, we can expect to see more of it wind up in the tails of our distribution curves.
At one point, about 2012, I tried to track multiple kits in one spreadsheet – it didn’t work well for me. The one overlapping segment may be the same, but all the rest of our DNA is much different. I now keep a separate spreadsheet for each kit I manage. And I’m primarily focused on my own, using the others as helpful tools to impute data for me. I have a spreadsheet for all of my known Common Ancestors, with a column for the TG when known. The spreadsheet is usually sorted by my Ancestor couple and their children and grandchildren – so it looks like a Family Group Sheet. A quick scan down the list shows if there are known TGs, and which Ancestor children have them. See: https://segmentology.org/2021/12/19/segmentology-common-ancestor-spreadsheet/
Jim
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Debby, Let me add that the Common Ancestor spreadsheet (mine is now over 8,000 rows) lets me sort for statistics. One of my next projects is to blog about the cMs for each generation – almost like my personal version of the Shared cM Project.
Jim
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Thanks for feedback. I look forward to what you come out with. Since I only match a fraction of kits I deal with, I’ve been in the mode of separate DNA Painter profiles for a bunch of key descendants. I like the visuals and can quickly view where a new match might fit. I can also incorporate matches on other platforms with chromosome browsers for additional insights. I’ve recently added the common spreadsheet to have better combined view of new kits, and can filter and compare different views of the matching segments, and view/update key DNA Painter profiles. I would love to have a better way of capturing key segments – I liked your previous column referring to them as atDNA Haplogroups! I’ve tried DNA Painter profile for the ancestral couple – but too much manual. I need to think more about ways to track some of the things I look for – “how often I discover X match triangulations as well as regular” – now the additional “triangulations that include descendants of more than 2 children of a couple at this distance”, “what percent of 4th cousins actually have matches” (I find it is much higher than normal projections because of the endogamy making McGuire charts less useful) etc. I look forward to any research on this kind of thing. Anyway, thanks again for giving me an additional way to question the data!
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Debby, OK, I did a sort of my CA spreadsheet by cousinship, and for each one tallied the # of Matches and total cM. Drum roll… 3C: 71cM; 65 Matches; 4C: 24cM, 121 Matches; 5C: 17cM, 365 M; 6C: 13cM, 1295 M; 7C 14cM, 95M; 8C 15cM 136 M. 5C1R: 14cM, 916 M; 6C1R: 12cM, 783 M. This reinfoced my opinion that the atDNA sweetspot is in the 5C-6C range.
Jim
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Very cool!
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Debby, Just be careful at DNA Painter that you paint only one side at a time – this would be easy for me, with just one kit. I’m not sure how it would work if we painted kits with more than one base – we’d need to insure the Painted segments were in the same TG…
Jim
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Agreed. The couple of times I’ve tried to develop one profile for an ancestral couple I’ve only ever used the “Unknown/Both”. I haven’t yet run across a case where I’ve had two triangulation groups on same part of a chromosome – in that case I’d probably just have two groups under “Unknown/Both”. X matches are the exception: sometimes I’ll be able to assume based on inheritance path among descendants. Anyway in these cases, I’m painting results of other efforts, not just loading match data which is the usual approach. But so far, the effort to record this way hasn’t seemed worth it.
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Ciao jim quel gruppo che ti ho parlato io con loro ho triangolazione.i segmenti di loro sono 7,5cm 9cm7,1cmecc e un gruppo di 11 persone più come ti ho detto l altra volta tre che non hanno pubblicato il loro browser perché e privato.mentre una corrispondenza hanno testato i suoi genitori ma e privato non posso vedere se abbinano ha me come il loro figlio.okay.pero questo gruppo anche quelli che non c’è il browser sono imparentati con quelli che il browser c’è la anno.anzi hanno una parentela certe volte più vicini tra di loro mentre io e qualcuno e un po’ distante come ti ho detto 4 cugino e due di loro 5 cugini.il fatto curioso e che loro sono tutti di origini di sicuro dai Balcani.Ho molte corrispondenze dai Balcani in questi giorni ho avuto altre due corrispondenze non appartengono ha questo gruppo ma anche loro sono dai paesi come Bulgaria e Serbia bosnia.e sono corrispondenze tra il 20,0%di DNA con un segmento il più grande di 12cm e un altra corrispondenza 18,9%di DNA con tre segmenti ma il più grande e appena 6,4cm.pero quest’ultima non ha il browser e privato per vedere dove abbino i segmenti.
Il sab 12 ago 2023, 21:08 Kevin Capodivento capodiventokevin6@gmail.com ha scritto:
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Kevin – The translation of your comments are mostly about small segments under 15cM. Great care is needed, as these might be false segments. I don’t have any experience with DNA from Bulgaria and Serbia. The translation also talks of matches with 20.0% DNA – that is a LOT of DNA from very close relatives – those are the ones you should focus on. Jim
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Ciao jim grazie per il fastidio.pero questo gruppo le corrispondenze di DNA sono le sequenti 9,4CM 1 segmento 14,2CM con 2 segmenti uno il più grande 7,5CM triangolazione,8,7CM 1 segmento triangolato,13,7CM 2segmenti il più grande 7,1CM triangolato,14,3CM 2 segmenti il più grande 7,5CM triangolato 15,1CM 2 segmenti il più grande8,5CM triangolato.8,0CM 1segmento triangolato.8,7CM 1 segmento triangolato.13,9CM 2segmenti il più grande7,5CM triangolato.14,8CM 2segmenti il più grande 7,5CM triangolato.9,0CM 1segmento triangolato.questi sono i dati di quel gruppo che ti dicevo loro hanno tutti balcanici.piu altre tre corrispondenze che non c’è il browser cromosomico.ti mando delle foto per farti vedere i segmenti.grazie Jim
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Kevin, For each Triangulated Group of segments, there is just one path for the DNA from an Ancestor down to you. The relatively easy part is establishing the TG – the hard part is often determining the genealogy link. It often takes a lot of hard work. Jim
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Ciao Jimmy io ho questo gruppo dispersione di 11 più altri tre ma non c’è il browser cromosomico per vedere se anche quei tre corrispondono anche loro sul cromosoma 16.questo gruppo si di 11 persone come ti ho detto l altra volta tutti stessa posizione con segmenti triangolati loro sono tutti dei Balcani io sud italia.non riesco ha capire se sono veri ho falso.perche una corrispondenza ho visto che hanno testato i suoi genitori ma e privato non riesco ha contattarli per vedere se uno dei suoi genitori corrisponde ha me per vedere se e vero e falso .perché se corrisponde un suo genitore allora non sono falsi.
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Kevin – without segment Triangulation (or phasing), I cannot tell if the segments Triangulate. You can try looking at the Shared Match (or ICW) list for each and see if there is a consensus among them or not. No consensus is a clue that the shared segment is false. Jim
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Hello Jim, I am afraid that I do not understand this at all. I have an example that I think disputes the first theory.  I believe that if I made the effort I could come up with more. Maybe I just have this wrong – it made sense until I read down to “Important Note” This is a segment on my Chromosome #1 – the earliest couple is William Bush (1753-1834) and his wife Sarah Radford. They would be my 4 great grandparents.  Here are the matches: Steven Carder – 10.7 cM – Wm and Sarah’s daughter CharlotteGeorge Boyd – 8.7 cM from Wm and Sarah’s daughter Martha/PatsyTess Jones – 7.6 cM from Wm and Sarah’s daughter RebeccaChris Inman – 13.7 cm from Wm and Sarah’s daughter SarahJeanette Pinger – 13.7 cM – from Wm and Sarah’s daughter Charlotte Attaching a snip – Best Regrds,Linda Hope
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Linda – A good example – let’s see… Do all 5 of these Match-segments triangulate with each other? What site are you using for comparisons? What child do you descend from? Jim
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Yes, they all triangulate. I am using Painter – these tests were done by different companies – 23andMe, Ancestry and FTDNA – I am from Charlotte Bush (1794-1851) She is my 3x great grandmother. There are 4 daughters of William Bush and Sarah Radford represented
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Linda – So you descend from Charlotte, and two of the Matches descend from Charlotte – correct? How are you comparing the segments to insure Triangulation – are they all at GEDmatch? Painter will show overlapping segments, but they could be from either side. There is the possibility that, although all the Matches descend from William and Sarah, one or more of them might also descend from some other Common Ancestor(s( – even on the other side. I don’t believe DNAPainter can Triangulate segments. Your case is fairly strong as all the Matches have genealogy on the same side, so segment Triangulation is an additional test to confirm that. Another point is the two small segments – 8.7cM and 7.6cM – these are in the danger zone for being false segments. One quick test is to analyze the Shared Match/ICW list for each of them. My experience indicates an ICW list that includes a number of Matches on the same line you are working is a stong signal that the Match Shared Segment is IBD (true). On the other hand if the ICW list is very short or all over the place (no clear consensus), this would be a strong signal that the Match Shared Segment is IBC (false). I suspect at least one of these small Matches may be false… Jim
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Pinger is on GedMatch but it is a FTDNA test – Boyd is not on GedMatch – they match each other when I use the matrix at FTDNA. Inman and Pinger match each other – both on GedMatch. Jones does not match Pinger or Inman. Carder is on 23andMe. I might be able to find a match there but doubtful as so few of those people tell you anything at all. But Carder does match me on four additional chromosomes. I need to remove Jones, but I am not certain what to do about Carder.
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Linda – You are working with a distant Ancestor, so small segments are needed. I agree with you to remove Jones. Note that all of the Matches you name may well still be genealogy cousins – they don’t have to share a DNA segment to be a cousin, and in fact most of our distant cousins will not share DNA with us. Between Inman and Pringer (Matches at GEDmatch) and your self, you’ve formed a pretty solid TG – and you have good links back to a MRCA. The task now is to find more Matches on this segment who also descend from William BUTH 1753. Some hard genealogy work ahead… Jim
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