Does Triangulation Work?

Sure it does! Triangulation is a tool to use with autosomal DNA. Let’s see how it might work:

  1. Does it work in grouping your shared segments?
  2. Does it work in culling out IBS segments?
  3. Does it work to define and map your ancestral segments?
  4. Does it work to insure that all Matches in a Triangulated Group have an IBD segment?
  5. Does it work in identifying Matches who all share the same Common Ancestor?
  6. Does it work for any size segments? – see more at: Does Triangulation Always Work?

The Big Picture

Let’s start with the Big Picture.  We take an atDNA test and the company reports a list of our Matches. We can also get Matches by uploading our raw DNA data to GEDmatch. Each of the companies compares our raw DNA data to that of all the others in their database, and uses their proprietary matching algorithm to generate a list of Matches. At 23andMe, FTDNA and GEDmatch, they also provide the shared segment information (Chromosome, Start Location, End Location, cMs, and SNPs) for each shared segment. For this discussion I’m only going to be talking about segments over 7cM, just to avoid any debate about smaller segments. Each of the companies have pluses and minuses that go along with their matching algorithm, but we are going to go with the list of Matches they provide to us.

So this is the data we want to work with using the Triangulation tool.

Ancestral vs Shared segments

Please re-read “What Is a Segment?” to recall there are ancestral segments – ones you get from an ancestor – located completely on one of your chromosomes; and there are shared segments – ones that the computer algorithm determines by comparing data on both your chromosomes with data on both chromosomes of another person.

Shared segments are either IBD or not-IBD (IBS)

Most of these shared segments are IBD – meaning they come from a Common Ancestor – common to you and your Match. Some of the shared segments are IBS – meaning they don’t come from a Common Ancestor; they are segments made up by the computer algorithm. We cannot tell which is which by just looking at the one shared segment. ISOGG has a very good wiki article about IBD and non-IBD (IBS) segments. The bottoms lines are:

  1. Shared segments (also called matching segments or Half-Identical Regions (HIRs)) 15cM and greater are IBD virtually 100% of the time.
  2. Shared segments under 5cMs should generally not be used in genealogical analyses [and in this post we are not considering shared segments under 7cM].

So for this blog post we will focus on shared segments from 7cM to 15cM as reported by the companies. And note that each of these segments is either IBD or IBS.

Triangulation Criteria

For Triangulation we find three sets of shared segments which match each other. This usually means you and two Matches have shared segments which overlap at least 7cM, AND the two Matches share a segment which overlaps the same area at least 7cM. This means all three of you have the same, long string of SNPs in the same location. This is Triangulation.

Usually Triangulated Groups (TGs) include more than just you and two Matches. They may include 5, 10, 20 or more Matches. Each TG includes all of the shared segments, and these triangulated segments determine the start and end locations of the TG, such that the TG includes them all.

My Experience with Triangulated Groups

I have over 5,000 different Matches in my spreadsheet, with perhaps 6,000 separate shared segments over 7cM. As a result of the Triangulation process, these shared segments have been placed into 4 categories:

  1. A Triangulated Group on my Dad’s side.
  2. A Triangulated Group on my Mom’s side.
  3. An IBS group (these segments overlap, but do not match, TGs on either side)
  4. Undetermined as yet

The TGs above cover 90% of my 45 chromosomes, and define 340 separate TG segments on my DNA. Most of the TGs are heal-and-toe (adjacent) to each other on each chromosome, with only a few gaps. All of my shared segments either “fit” into (overlap within) one of these TGs or they are IBS (or they are undetermined).

TGs Form a Chromosome Map

The key point here is that these TGs map my chromosomes into specific segments. Each of these specific segments comes from an Ancestor. Similarly your chromosomes are divided into specific segments, defined by crossover points from each generation. Re-read Bottom-up and Top-Down for a refresher on how crossover points and segments are formed. Within each such ancestral segment, defined by start and end locations, each of us will have a continuous string of SNPs – usually thousands of them. And each such ancestral segment comes down a specific path from a specific ancestor to us. On this point endogamy does not matter. In the bizarre extreme, all of your ancestors in one generation could be the same man and woman, but each of your ancestral segments only came from only one place in your Tree, and down one path to you.

IBS Segments

 Most of your shared segments will be IBD and will form TGs. But some shared segments will overlap the segments of a TG but they won’t match any of them. On either side. These shared segments are clearly IBS. If they were IBD – from an ancestor – they would match overlapping segments in a TG.

Are All Segments in a TG IBD?

So one of the arguments for Triangulated Groups is that if three people all match each other on the same segment, the shared segments must be IBD. We have three pairs of matches, each pair with the same long string of SNPs. We have all three companies with proprietary algorithms that try to insure their Matches are IBD. We have TGs that are mapped on our chromosomes, and know that some ancestor provided that segment. It sure looks like the shared segments in these TGs have the same SNPs that our ancestors passed down to us. This is even more compelling when there are several, or more, shared segments which Triangulate and form a TG. But are we sure every segment in a TG is IBD?  Read on for some possible exceptions.

Some Areas to Look Out for:

  1.  If you have only one Match (Match1) who matches a number of your close relatives in an apparent TG: You and Match1 might share an IBS segment (based on Match1’s segment being false); and then Match1 may well match all of your close relatives who have the same ancestral segment you have. For this reason observe the caution that TGs should be formed with widely separated cousins – the wider the better. Another test is to find other Matches (not closely related to Match1 who Triangulate with you and see if they match Match1. All Matches in a TG, who overlap enough, should match each other. If they do not, then an analysis should be done to weed out any potential IBS segments.
  2. You match several other Matches who are closely related to each other in an apparent TG: you might have a false segment and may well match all of the Matches who share the same good ancestral segment. As in the previous paragraph, it’s important to form a TG with widely separated cousins. The test here is to look for other overlapping Matches for this segment area. If this is an IBS TG, the other Matches will not also match the Match family. Also, you do have a true ancestral segment for each area of your chromosomes. If several related Matches all match you in one segment area, and your segment is false with them, you should be able to form two other TGs (one from each parent) based on your ancestral segments compared with other Matches.
  3. Another argument used to debunk Triangulation, is endogamy. The theory here is that due to endogamy – some of our ancestors being the same person – the same ancestral segments are floating around and TGs may be formed with different ancestors. In theory, this is possible – in practice it is improbable. In the first place, endogamy means the two ancestors who are the same person actually had a Common Ancestor. So in fact the TG shared segment really did come from Common Ancestor, several more generations back. With each generation going back, the probability of a match is divided by 4, or 16 for the two generations involved in a first cousin endogamy. Clearly it is much more likely that our Matches in a TG are from a closer cousinship.

Also, based on my chromosome map, the ancestral segment I got for each TG is from a specific ancestor, down a specific line of descent to me. It has a specific string of SNPs that we generally think of as unique. Is it possible, in the 7-15cM range, for a Match to have exactly the same string of SNPs from a different ancestor? With random DNA almost anything is possible, but the premise of autosomal DNA is that this would be very rare. If it did occur, the shared segment would technically be IBS, because it was not identical because of descent from a Common Ancestor. But, we might have to leave the door open for this possibility.

Back to a Big Picture Thought

The number of people taking an atDNA test is about doubling every 12 months. If this continues, I’ll have 10,000 Matches, with shared segments, by this time next year; and 20,000 Matches by the end of 2017. My chromosome map has pretty much been determined (I am now focused on determining the correct Common Ancestor for each TG). A doubling of Matches means a doubling of each TG every year. The point is that if we assume 80-90% of our Match segments are IBD (I actually believe it’s closer to 95%), all of those IBD segments are being added to my existing TGs. Couple this with the fact that most of our Matches are beyond 5th cousins (I believe most of our Matches are actually 6-8th cousins, and some beyond). Even if a few of the Matches in our TGs turn out to be IBS, we are still getting a great influx of true cousins into our TGs.

So to summarize:

  1. Do TGs work to group your Matches? Sure! Instead of the long list of miscellaneous Matches reported by the companies, you can form Triangulated Groups. See Benefits of Triangulation.
  1. Do TGs work to cull out IBS segments? Sure! Many of your 7-15cM segments will not triangulate with any overlapping TG, indicating those “shared” segments are probably IBS. As noted above, not all of the IBS segments may be identified this way, but many (I think most) will. This is progress – it’s an improvement over the list you get from the companies.
  1. Do TGs work to define and map your ancestral segments? Absolutely! It’s hard work, but an easy mechanical process to define the TGs with start/end locations; and only a little genealogy with known relatives is needed to assign them to maternal and paternal sides.
  1. Do TGs work in insuring all Matches in a TG have an IBD segment? Almost all of the time, and there are ways to find and test suspicious shared segments.
  1. Do TGs work in insuring all the Matches in a TG share the same Common Ancestor? This is a tough one because it’s not possible to rule out some outliers. As noted above, if you carefully form the TGs, the Matches should come from the same Common Ancestor. We have lots of examples of Matches in TGs who do share the same CA. It’s very hard to prove that an IBD segment is really from a different ancestor; and I haven’t seen a single case of it so far.

Your ancestral segment in each TG does come from a specific ancestor of yours, and your cousins from that Ancestor with that segment will match you on it in that TG. As several of us have suggested, to determine the true Ancestor for a TG, you need to “walk the segment back.” This means finding cousins at various levels in each TG – a 2nd cousin, a 4th cousin, and a 6th cousin who all have the same segment and ancestral line. This is often hard, but the number of people taking an atDNA test is doubling annually, and more of these intermediate cousins will gradually show up in our Match lists and TGs.

Bottom line for me: Triangulation is a powerful tool.

 

11B Segment-ology: Does Triangulation Work? by Jim Bartlett 20151019

24 thoughts on “Does Triangulation Work?

  1. How far back in time have you identified an ancestral segment? How many people in that TG? Do you think matches pre-1600 are just from the population pool? I.e., do you have a cut-off date?
    Many thanks. We appreciate your sharing with us.

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    • Caith, I’m not sure I’m ready to claim absolute victory on any of my segments yet. I’ve identified Common Ancestors with some 10th cousins and a few 11th and 12th cousins. Are these the correct Ancestors for the segment? I can’t really say. I’m trying really hard to get consensus on as many TGs as I can. I have between 2 and 100 Matches in my TGs. I’m guessing that the TGs with many Matches are from Common Ancestors with large families and many descendants who have tested. Some may be large because they come from more distant Common Ancestors and thus there are many more descendants. 3/4 of my ancestry is Colonial Virginia, so I expect some of those TGs to have large families and be more distant. Another good metric would be the average size of segments in a TG.
      I believe there is a population pool, but I don’t believe any ancestors get matching segments over 7cM by random events. From the drinking water? Not hardly! If they have matching segments over 7cM, it’s almost certainly because they got them from a Common Ancestor. That’s the result of endogamy – cousins marry and their descendants have the same ancestors in different places in their Trees. These ancestors have the potential to pass the same segments down to us through different paths. But now, instead of say 8th cousins, we are talking about 10th, 11th or 12th cousins passing down these “population” segments. A 10th cousin has only 1/16 the probability of an 8th cousin matching – and an 8th cousin is already long odds that has to include a pretty “sticky” segment. To me, “population” segments mean endogamous segments, which just means Common Ancestors farther back. Possible yes, probable no. IMO.
      At this point I don’t have a cut-off date. I have stretched my Tree out to 13 generations wherever I could. The problem is finding other Matches with such Trees. We all have brick walls, so it’s impossible to say any Common Ancestor is correct. Right now I’m just trying to find Common Ancestors where I can. I don’t know where the limit will be. It depends partly on the segment threshold. A 15cM threshold will make for closer/wider TGs; a 5cM threshold will find more TGs out on the long quills of our porcupine chart. I don’t have many pre-1600 Matches, and those are suspicious – I’d need a lot of corroborating intermediate Matches with closer cousins.

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  2. I, too, am primarily of Colonial America descent. My tree is not as well documented as it should be. As an exercise and out of curiosity , I have been through EVERY tree of my DNA matches at Ancestry and have logged every tree match, knowing, of course that is not a DNA match, without segment matching. Augustine Warner born1652 has been in the trees of my dna matches 58 times. What does that mean without paper documentation. Probably nothing.

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  3. Thank you for this very interesting article.
    I use Family Inheritance: Advanced to process the genomes of my DNA cousins and I record it on a Word document. My “clumps” are given different coloured surnames to distinguish them from one another.
    I find that 23andme is a bit optimistic about how closely related we are, and because I’m Australian most of my matches are said to be 4th or 5th cousins, but in reality are more likely to be 9th or 10th cousins. My paternal great grandfather was an American so all his ancestors were from Massachusetts, and their descendants would have migrated all over America.
    I have discovered about 15 people with whom I know we share common ancestors, and so I can tell those other matches who their common ancestors are. It may not help some of my many adopted DNA cousins but I tell them that at least they now know who to aim for, and I try where possible to bring the common ancestors’ descendants as far down as I canun my tree. Given the number of children they all had in those days it’s no mean feat!

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    • Merilyn, Thanks for the kind feedback.
      I think all of the companies have skewed cousinship estimates. I think this is because they use mathematical models which underestimate the true nature of “sticky” segments. For some reason, I believe “sticky” segments survive longer than was expected and so we see a wide range of cousinships with the same sized segments. As a result, I don’t pay much attention to the cousinship estimates beyond 3rd.
      It is going to change genealogy significantly as we nail down Common Ancestors to specific segments. Then, as you correctly point out, we can tell our Matches (including adoptees), who their ancestor was. This has also worked for me in a number of TGs. There is always the chance that someone will descend from a more distant ancestor, but even then our clue is a great “pointer” in the right direction.

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  4. Jim, thanks so much. One thing I really like about your blogs is that you state everything in a simple enough way that even a beginner at DNA research can understand it. There was one sentence, though, that I read half a dozen times and am still not sure I’m catching the intended meaning — this one: “If several related Matches all match you in one segment area, and your segment is false with them, you should be able to form two other TGs (one from each parent) based on your ancestral segments compared with other Matches.”

    It’s the third word — “related” — that I’m stumbling over. What does that refer to? Related to whom and in what way?

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    • Gary, Thanks for the positive feedback – that’s exactly my goal;>j

      Sometimes I triangulate with a close family group – several family members have tested. They will all have lots of identical DNA. So when I match them all, this should be counted as just one of the Matches needed for Triangulation. This is because my part of the shared segment may be the made up segment. And when it matches one of the people in the family on their good segment, it may well match several others with the same good segment, giving a false sense of Triangulation. So when I check to insure that my Matches match each other in a TG, I’ll note if they share hundreds or thousands of cMs, and treat them as just one Match in forming a TG.
      Maybe I should have said “closely related” or “interrelated”

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      • I’d like to restate my understanding of this. Say I’ve found three women who are sisters (not my sisters). They match each other on a segment and I also match each of them on the same segment. I should consider them one additional match in this potential TG, so in reality I’ve got two “matches” in this potential TG (me and the collective “them”). Unless and until a third party is added to this group, I cannot tell based on the four of us if we are a true TG or just random.

        Is my understanding correct?

        Thanks Jim!

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      • Judy – You are correct. You may well have a Common Ancestor with “them”. You may well have an IBD (ancestral) segment with each of them (if your shared segments are over 15cM, don’t worry, they are almost certainly IBD and from a Common Ancestor). If the shared segments are below 15cM, you want to Triangulate to cull out potential IBS (false or random) shared segments.
        However, like at AncestryDNA where we don’t have any segment info, you can proceed to look for a Common Ancestor with the trio. No harm in looking…. I look for Common Ancestors with every Match I can;>j Even if we don’t share an IBD segment, we can still be cousins – just like in the (not so) old days – before DNA.

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  5. I’m not sure if I understand the endogamy part. That the argument used to debunk triangulation is endogamy. At least I haven’t heard that. I’m not certain what you mean about this theory, but I guess this goes back to the different types of endogamy? The ones where people marry relatives, so making them have the same common ancestor further back? I know for me, many of the segments come from the same ancestor(s) multiple times. Multiple to me means more than 3 times. But in the case of this where we’re trying to describe endogamy, I’m referring to thousands of years of this happening.

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    • Kalani,
      I think that argument says that endogamy means some of your ancestors are the same individual, and if the Common Ancestor is this person, which ancestor are you talking about. Another argument is that the same individual, in two places on your ancestry, could have passed down exactly the same segment through two different lines and they both show up in a TG. Although technically possible, the odds are virtually zero.

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      • Thanks for clarifying. I had a known 4th cousin tested and compared her to our 1/2 3C1R. I was surprised because I expected us to have the same common segments. But we did not. But knowing that 4th cousins there is a 50% chance of us sharing DNA, between she & I, we share 104.24cM (FTDNA) total, but our largest segment is 15.84cM. With my brother, 114.72cM, same size for the largest segment – 15.84cM. With our mother (3C1R) they share 217.98cM total, largest segment 18.90cM. With the other cousin (1/2 3C1R) she shares with our 4th cousin 73.01cM (largest 10.15cM) but with me 130.44cM total (33.24cM largest). I really thought we could map our common ancestor but we can’t. And the size, it is just misleading. I’m not even sure how to go about triangulating with our numerous ICW matches when it comes time to figuring out our common ancestorS.

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    • Donald,

      The shared segment info is the Chromosome, Start and End Locations, cMs, and SNPs that describe each segment you share with a Match. You find this information through a Chromosome Browser at FTDNA, in FI:A at 23andMe, in the one-to-one utility at GEDmatch. AncestryDNA does not reveal this info.

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  6. Hi Jim,
    I’m always a bit confused by triangulation- with my family, I do not match a known cousin, who I’ll call “H” for simplicity here, but we do both match another cousin “J”. We both match this cousin on different large segments. We also can prove that “H” matches other descendants of the same common ancestor (who are not on my match list) but they also connect on different large segments. That being said, when we look at how I relate to “J”, and how he relates to “J” and the other descendants, we do have a tiny segment (2.4 CM, sometimes a tiny bit less), in common. I always feel like I’m missing something, or maybe that we relate in a different way than we thought? I find it all very confusing! Would this be a common situation? If you have an email I can gladly share the chart with the matching segments so it might make more sense. Thanks for any insight!
    Amy

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    • Amy, Your situation with different cousins is normal. No need to send a chart, I’ve seen this many times. We don’t show up as a Match with all of our real cousins. You will only match 9 of 10 3rd cousins and only half of your real 4th cousins. The way the DNA trickles down from our ancestors, it’s kind of amazing that we share at all beyond 4th cousisns. But we have so many distant cousins, that we will share with a lot of them (but only with a small percentage of them). My strong recommendation is to not fool around with any small segments below about 7cM. Most of them are false segment, and it really muddies the water. Stick with segments over 7cM (some people only use segments over 10cM or even 15cM, just to insure they are all true Segments and the Matches are true cousins. Triangulation will cull out almost all of you false segments.

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      • Thanks Jim! Is it normal to match on different chromosomes, if we all descend from the same couple? For example, I match cousin J on chrom. 4 (41 CM), but my cousin H matches him on chrom. 17 (29 CM), and he matches other descendants from the same couple on different chromosomes as well. To triangulate, don’t they all have to match on the same segment on the same chromosome? I suppose that’s what’s confusing me, and I’m having a hard time articulating it 🙂 Thanks again.

        Amy

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      • Amy,Yes. Read my Top-Down post. Earilier, I was counting up the number of segments I may have gotten from my ancestor Peter RUCKER – there were 16 of them, each with a different mix of cousins – some of the closer cousins repeated, but most were Matches with just a sing

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      • Hi Jim, what if I have a group of say 10 people who all match me and each other in a particular position on a particular chromosome. Some will have small segments but others have decent sized segments. Couldn’t the people with small segments be included in this group who will all share common ancestors give or take a generation or two?

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      • Merilyn,

        It might work. We still don’t have a good way to confirm if triangulation works on segments under 5cM. I’ve been adding 5-7cM segments to my spreadsheet. The vast majority of them do not triangulate, so I’m inclined to keep the few that do. But I color them a little differently to remind me to be cautious with them. The jury is still out, but there is no harm in leaving them in as long as you don’t make sweeping decisions based on them.

        Jim – http://www.segmentology.org

        >

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