When working with Matches on one side (Maternal or Paternal), segment Triangulation is a snap. Overlapping segments are all you need! The overlap should be at least 7cM, and more is better.
The basic rules to form Triangulated Groups, were designed to insure your overlapping shared DNA segments were on the same side – in other words on just one of your chromosomes. This means, from your viewpoint, the overlapping segments were both (or all) on your maternal *or* paternal chromosome. It didn’t matter which side it was on for your Match. You can have lots of shared segments on one chromosome, but some may be on your maternal chromosome and the others on your paternal chromosome. It is virtually impossible for Match A’s shared segment on your maternal chromosome to also match Match B’s shared segment on your paternal chromosome. So the requirement is/was to compare Match A and Match B to insure they match each other – and are thus on the same chromosome with you. *IF* you already know Match A and Match B are on, say, your maternal side, then their shared DNA segments with you would be on your maternal chromosome, and there is no additional need to compare them to each other – they Triangulate.
I am sure, in the grand scheme of genetic genealogy, that an occasional glitch could occur. I’d estimate this as way less than 1% probability.
FTDNA has maternal and paternal buckets which appear to be pretty accurate. If the companies designated a “side” and allowed us to filter Matches based on that side, it would sure speed up segment Triangulation. Just look at a spreadsheet for natural crossover breaks in each chromosome.
In the meantime, if you can designate your Matches as Maternal or Paternal in some way (compare to a parent’s test, ethnicity, shared matches, etc.), you can use that info to filter your Matches and ease the segment Triangulation process. There’s still a lot of work to do, but this should ease the process some.
[10E] Segment-ology: Triangulation on a Side Is a Snap by Jim Bartlett 20230730
segment-ology 
Ciao io con questo gruppo di persone ho stesso segmento su chromosome 16 e altri sono riuscito ha capire da che lato corrispondono sempre sul 16 mentre con questo gruppo sempre sul cromosoma 16 e mistero non so Como sono imparentato con loro.sono cugini di quarto grado .qualcuno di loro ha fatto il test anche i suoi genitori perché ho una corrispondenza con il loro figlio di 5 anni mentre io ho 51 anni mentre i genitori del bambino nel loro sito ho visto che hanno fatto il test però e scritto analisi del test DNA privato.non hanno pubblicato i loro test hanno solo pubblicato il test del loro bambino.per capire se corrispondo ha uno dei genitori CSI per vedere se era vero ho era falso .perché con il loro bambino di 5 anni ho una corrispondenza di 14,8% non e molto e però lui e un bambino capito.come ho detto un altra volta nel mio lato materno ne mia nonna ne mia bisnonna non si sa chi sia il loro padre perché la mia bisnonna e padre incerto mentre sua figlia che sarebbe la mia nonna ha lo stesso cognome della madre che era figlia di padre incerto .capito?
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Yes. This is the problem for genetic genealogy – finding the correct Common Ancestors/ There should be a consensus in each TG, because it is DNA from a particular Ancestor. Jim
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Andreas and Louis and others… I’ve stuck my neck out and posted on atDNA Haplogroups – please be gentle with me – and please post your thoughts – we are all in this genetic genealogy journey together.
Jim
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Ciao volevo sapere che vuol dire che abbiamo una triangolazione DNA con antenato comune e uno ho una coppia di antenati?un triangolazione su un segmento che vuol dire un antenato ho due antenati?
Il lun 31 lug 2023, 14:06 Kevin Capodivento capodiventokevin6@gmail.com ha scritto:
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Sorry this was a delayed approval. I got this translated from Italian to English – hope it is accurate.
Kevin, Yes, the Triangulated Group (TG) segment usually goes back to a couple (a husband and wife) and we find cousins who share this same couple. We call the couple the Common Ancestor – but we don’t really know which one of them is the one who passed down the DNA segment to ourselves and our Matches. To do that we have to find a Match who is a more distant cousin – back through the husband or the wife. Hope this helps, Jim
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Ciao Kevin,
normalmente un antenato comune è una coppia in quanto è impossibile stabilire se il DNA abbia avuto origine dal maschio o dalla femmina di detta coppia di antenato comune. Per identificarlo, dovremmo tornare indietro di un’altra generazione, il che significa che identificheremmo una nuova coppia MRCA.
Un antenato comune più recente, tuttavia, può essere una singola persona, se quell’antenato aveva due o più partner attraverso i quali il DNA è stato trasmesso a individui sottoposti a test del DNA.
Spero che questo risponda alla tua domanda.
—-
Hi Kevin,
normally a common ancestor is a couple as it’s impossible to tell if the DNA has originated from the male or female of said common ancestor couple. To identify this, we would need to go back one more generation which means we would identify a new MRCA couple.
A most recent common ancestor however can be a single person, if that ancestor had two or more partners through which the DNA was given down to DNA tested individuals.
Hope this answers your question.
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Genealogia
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Buongiorno io ho un gruppo di persone che abbiamo stesso segmento su cromosoma loro hanno dei segmenti triangolati con me sono cugini di quarto e quinto grado.non riesco ha capire come sono imparentato .nel mio albero mi manca il lato materno di mia nonna che il padre non so chi sia e sua madre era figli di padre incerta.e possibile una connessione.con loro? Io sono italiano sud Puglia Basilicata e Campania loro sono tutti dai paesi dei balcani
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Great content as usual, Jim. Some additional comments from my experience with my app “Your DNA family” which automates the process of assigning triangulated groups and also the assignment of paternal/maternal sides:
There are some DNA matches (which I call “bridges”) which are very long and can cover 2 or even more adjacent TG’s. Identifying them is crucial as otherwise our TG’s become “too big”, meaning the actually consist of more than 1 TG and many DNA cousins will only match on the left side whilst others will only match on the right side of the TG.
Researching and extending the family trees of our DNA cousins is crucial in identifying MRCA’s (most recent common ancestors) that they have with other DNA cousins in the TG or even ourselves. Don’t just work on your own family or try to find how you are connected to them. By identifying MRCA’s between 2 DNA cousins in a TG you will identify a branch of their ancestors through which they have inherited the ancestral DNA segment.
Include more and more DNA cousins (the “Walk the ancestor back” method of Jim) to eventually identify how you are related to them and to ultimately identify the common ancestor whose DNA you all have inherited.
If you’re interested to learn more about automatic DNA segment triangulation (to address Jim’s “there’s a lot more work to do” comment) check out a recent video of the method from a presentation to the Portland ISOGG group: https://youtu.be/LgEPldZhG7s
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Andreas – thanks for your feedback and insights. Yes – at the end of the day, we should have TGs filled with many Matches and their segments (the number of Matches usually depends on who tests which is a function of how large the family is, and if it’s in USA where most of the testers are). And yes, closer cousins may bridge more than one TG. When you think about Chr 01, for instance, and perhaps 20 TGs spanning that Chr, our parent gave us DNA that spans them all. And a first cousin may span several TGs, But in general, I think this plays out pretty quickly, and we generally don’t see 4C or beyond spanning two TGs. However, there is always that little tail on the distribution curve of the data that reminds us that many things are possible with DNA. Jim
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Hi Jim: one of my fundamental concerns with matching is what if my dna matches others simply through chance. I don’t know if the math with this has been worked out. Also, I don’t think triangulation can help with this because if one has a matching sequence by chance – and if there are many others with the same sequence then they will all triangulate. Wondering if you have any suggestions to resolve my conundrum?
Thanks,
RS
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Hello RS, DNA segment triangulation will identify these DNA matches by chance (IBC) as they won’t qualify for triangulation. Whilst there are some areas of the chromosomes (eg on a specific part of chromosome 9 – see this video for explanation https://youtu.be/LgEPldZhG7s ) that have a higher number of IBC matches, they are usually small as the DNA testing companies are now doing a great job of sorting them out (meaning, they won’t be shown as matches to you).
And no, if they match you by chance it doesn’t automatically mean they match all other DNA cousins by chance too. It’s a bit complicated topic but it’s basically due to using unphased data, where the matching allele jumps back and forth between the paternal and maternal side. While this might lead to a “match” with yourself, it won’t work with the other DNA cousins as they do have other values on the opposite side as you (otherwise if you would have the same values on both allele it would be a “Run of Homozygosity” (ROH) which indicates a common ancestor between both parents.
Hope this helps, Roberta Estes has an excellent blog article on IBC matches that probably explains it better than my short comment.
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RS – I’m not sure I understand your question entirely. Segment Triangulation generally culls out almost all false DNA Matches (which are almost all below 15cM) – in other words, if each Match in a TG matches several others, on same/overlapping segments, there is almost no probability of a chance DNA match. If you question is that you may find an Common Ancestor who is not the CA linked by DNA – that is an issue. Within a TG we strive for a clear concensus of the ancestral line and the CAs along that line. It is very possible for a Match to be related to me more than one way, so we rely on the concensus among Matches. Ususally, when a consensus is found, I can go back to the Match who is an outlier and research his/her Tree some more to find the consensus CA. Hope this helps, Jim
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Jim: I think RS is referring to a case where both of a persons chromosomes are by chance matching all the SNPs in a triangulated segment. In this case the person will appear to match everyone in the TG even though he is matching by chance. RS is correct in his worry and I don’t think Andreas’ answer is addressing this specific situation.
RS: The one good thing about this situation is that it is less likely than two people matching by chance because two people can have either chromosome of one person matching either chromosome of the other person. Whereas this case must have either chromosome of the one person matching the TG exactly. Since matches by chance between two people are unlikely for segments over 15 cM, matches by chance with a TG will not happen maybe over 7 cM. My 7 cM is an educated guess because I’ve never seen an analysis to determine this value.
I wrote an article about this a few years back: Triangulation does NOT mean IBD: https://www.beholdgenealogy.com/blog/?p=2284
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Louis, Good to hear from you – I’ve read your article, several times. Most of our genetic genealogy community is in pretty good agreement that shared DNA segments over 15cM are IBD (true segments on one chromosome). So the discussion of IBD , or not, really falls on shared segments under 15cM. My experience is that segment Triangulation identifies (culls out) most of these, because they don’t Triangulate. Yes, a few may slide in. However, I also contend that our own segment in the TG is IBD. This is further demonstrated as we Triangulate all of our shared segments and have TGs that literally cover each one of our 45 or 46 chromosomes. These TG segments are clearly my DNA – IBD for every TG. This also provides a good base for each of the Matches in a TG – if they also match all *their* other overlapping segments in the TG, their own segment is also IBD. [In fact I am currently writing a blog post: Triangulated Groups are atDNA Haplogroups – a unique string of SNPs that came from an Ancestor. I’m finally coming around to Tim Janzen’s dream project.] Whether we build TGs using your Double Match Triangulation method, or DNA Painter, or whatever method – the resulting string of SNPs is personal to ourselves – it mirrors our own DNA – it’s IBD. A few of the Match segments, under 15cm, may not be IBD (and that can be resolved by the Match or by us, if it is critical to our analysis). But for the most part, a few Match segments are not critical – we are looking to “Walk the Ancestors Back” along an ancestral line – the line on which the TG segment was passed down to us. BTW, my segment spreadsheet is now over 20,000 rows divided into 372 TGs, and I’ve pretty much stopped adding shared segments under 15cM… My Chr Map of segments is done – the issue now is finding Common Ancestors among my Matches – now *there’s* the rub.
I hope this also addresses the concern RS raised.
Jim
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Jim, you wrote:
“In fact I am currently writing a blog post: Triangulated Groups are atDNA Haplogroups – a unique string of SNPs that came from an Ancestor. I’m finally coming around to Tim Janzen’s dream project.”
This unique string of SNP’s is called a microhaplotype (they are actually several different microhaplotypes that are adjacent to each other) and it’s indeed being preserved over many centuries, further back than our genealogical timeframe.
However, they are unique in such a way that all DNA cousins in a triangulated group have inherited them from the common ancestor.
I’m very much looking forward to your upcoming blog post, as always.
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Louis, I’ve read your blog entry and I know Debbie’s argument, however I disagree with her and your view.
In your blog post you write “If other people also triangulate with them over the same segment, then all people must match all the other people on the segment.” – that’s not necessarily true when you apply a threshold of centiMorgan or SNP’s. In reality the match exists, yes, but it might be under the threshold that the DNA testing company uses (or is the current standard, like 7 or now more 8 cM).
You also write “You could theoretically have 3 people who all match each other zigzagging back and forth between their parental chromosomes” – as I pointed out in my first answer this is completely impossible on the current thresholds used. Even IBC between two is impossible over such a number of SNP’s as it would mean that both person have the same allele on their opposite side (their other parent).
“One of the People Match by Chance” – I’ve pointed it out in my earlier answer that this isn’t possible, again, reference to centiMorgan threshold of 8 cM is made. I challenge you and Debbie to come up with one example of real world data to support that view.
Some say there’s also a possibility that Alien lifeforms exists somewhere, yet no one has proven them.
“Matching Segments are on Different Parental Chromosomes” – Blaine’s theoretical example. Unfortunately very few people have code a matching algorithm with raw DNA data, both Debbie and Blaine don’t have any experience to my knowledge with that level of DNA.
The probabilities just wouldn’t work out, in the same way as on the earlier point of IBC matches. Yes, I can construct examples with let’s say 10 SNP’s which is already very unlikely to happen in nature (because constructed examples completely disregard that microhaplotypes exist as they are inherited as a group of SNP’s due to their proximity to each other).
Now extend these constructed matching segments to a length of 700-800 SNP’s and you understand that the probability for such to happen has long been close to 0 at a much smaller number of SNP’s.
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@lkessler : you captured my concerns perfectly.
@Andreas: glad to hear that your research indicates that the possibility of exact IBC matches are small to none at 7+cM. I was wondering if you have come across any references where this analysis was done. I have been looking for something like that for a while know.
Thanks!
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Hello RS,
not sure if there’s a scientific paper about it, I doubt it. Why would someone work on this, it wouldn’t really help you in getting a grant.
My experience is based on currently 6,598,205 comparisons between 2 DNA kits that my users and I myself have done. Not a single comparison or TG was being flagged as an IBC that triangulates.
But many common ancestors and MRCA’s have been identified to verify that Jim’s “Walk the ancestor back” method works when working with DNA segment triangulation.
My app is founded on DNA segment triangulation (because it’s fully automated) and the results that my users have achieved are proof enough that this method leads to many discoveries and insights into our ancestors, thus verifying the genealogical work (and in some cases indicating that the genealogical work isn’t correct due to NPE events or other mistakes).
Now others like to use other methods like clustering but as Louis Kessler has pointed out in the aforementioned blog post this has the caveat that B vs C matches on opposite strands aren’t being identified (the shortcoming of In-Common-With).
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Andreas,
Aren’t Y-DNA results defined these days on a single SNP?!! mtDNA defined on just a few SNPs? Our shared DNA segments should have a minimum of 700 SNPs and TGs ususally have over 1,000, often much more. A DNA test looks at 700,000 SNPs, on each side. I have roughly 186 TGs on each side – the average would be 3,763 SNPs per TG. Simple math with actual data which can vary somewhat – I’m just trying to paint a big picture (not focused the outliers). 3763 SNPs is a pretty long, unique, segment string. That’s a haplotype for my DNA – a full group of them in a TG would be a Haplogoup. Y and mt, even allow for variances in a Haplogroup. I don’t know of a study that indicates how far back a 3763 SNP segment can go, but from my experience it’s in the 4xG to 7xG range – give or take. I’d say within 3 centuries, not many, and, for the most part, within a genealogical time frame. It’s hard to prove it one way or another until we get full Chr Map linked to Most Distant Common Ancestors. As you well know, some of the smaller Match segments in a TG can be from more distant Ancestors (right or left side in the TG segment – back on the paternal or maternal parent of the MDCA). So, yes, it’s likely that some Matches will be cousins beyond a genealogy horizon, but the full TG MDCA will be closer, IMO.
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Hi Jim,
first of all the number of SNP’s is “only” 3,763 per TG in your spreadsheet due to not testing every single position of the genome. As your wrote, some 650,000 – 850,000 SNP’s are tested with the arrays vs a WGS (Whole Genome Sequencing) DNA test which tests all 3 billion+ markers.
So in comparison to the very few Y-DNA and mtDNA markers we have a lot more for the autosomes.
I agree with the range of the common ancestor, the 7 (or now more 8 cM) threshold is aiming at the genealogical timeframe but based on my actual own research as well as those of my users it’s more likely to be a bit further out (see also statistics provided by Coop and Speed & Balding).
So yes, I think all those SNP’s would add up to a unique “fingerprint” of DNA variety which is coming from a common ancestor but it also consists of even smaller DNA segments which are from the many ancestors of the common ancestor. Which is why we can identify matching segments (albeit usually very small ones) with ancient DNA samples.
These smaller segments within our unique segment that is representing our TG can go back to many (adjacent) microhaplotypes, which BTW are used by the DNA testing companies to identify where our ancestors are from.
Hope it’s clearer what I meant, we’re absolutely aligned on our view!
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Great explanation!
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