Pile-ups

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What’s all the buzz about “pile-ups”?  In my mind there are three kinds of pile-ups: small, medium and large. They are different, so it’s important to understand each one. In this case Goldilocks should prefer the large pile-ups, but let me go through my views of all three kinds.

Alert: This post contains my opinions about small pile-ups and AncestryDNA (based on my own experience) so you should make your own judgments.

Background

I think the two keys to success with autosomal DNA lie in a robust Tree (as many ancestors out to 13 generations as possible) and as many Match-segments as possible (including as many close relatives as you can get). I spent about a year expanding my Tree as best as I could, and then posted that GEDcom in several places. I’ve tested at all three companies and use GEDmatch.  I put every single shared segment I can find over 7cM into my spreadsheet, and I periodically run a Quality Control check against a fresh download to pick up any missed Matches or segments. I currently have 5,000 different individuals with segment data in my spreadsheet, and have determined a Common Ancestor (CA) with 309 of them.

I have compared virtually every segment against other overlapping segments, and formed Triangulated Groups (TGs) that cover over 90% of my 45 chromosomes. It is now rare for me to get a new shared segment that changes my chromosome map in any way. This process has provided some insights on medium and large pile-ups.

Pile-ups

My definition of pile-up sizes:

  1. Small is smaller than 5cM
  2. Medium is 5-10cM
  3. Large is greater than 10cM

Small pile-ups – by my definition, these pileups are composed almost entirely of IBS shared segments. When AncestryDNA first rolled out their autosomal DNA test, their threshold was 5Mbp. This threshold included many shared segments well below 5cM, and resulted in many thousands of bogus Matches. To their credit, they provided a caution about these. When AncestryDNA revised their threshold to 5cM, many of these Matches went away. Part of their explanation was the elimination of “pile-ups”.  I agree that these “small pile-ups” should be eliminated. And when they reset their threshold to 5cM, that should have eliminated this problem. However, their explanations continue to stress the elimination of “pile-ups”. I just hope they don’t also toss out Matches in larger pile-ups – throwing the baby out with the bath water.

Medium pile-ups – 5-10cM range. As I gathered as many segments over 5cM as I could and sorted them in my spreadsheet, I noticed a few areas that had many such segments, all in a very narrow chromosome area. Very clearly a pile-up! Virtually none of them matched each other, although they had almost the same segment start/end locations. And there were a lot of them – many more than in large TGs.

In discussions on various email lists, we compared notes, and found that most of these areas were unique to our own experience. In general they were not due to some common feature of most human genomes. A notable exception to this blanket statement is the HLA Region on Chromosome 6 – roughly from 29.8 to 33.1Mbp.

However, most of the other areas were not tied to known issues like the HLA Region. In my analysis, it was not possible for me to link these to one parental side or the other. The fact that these areas include so many IBC segments indicates to me that it’s the combination of both of my chromosomes (maternal and paternal) that allows the “matches”. It’s the unique combination of alleles in these small stretches of DNA that make matching much easier. And this unique combination is only in my genome. On chromosome 18, I have 307 segments in the 7 to 11 cM range. They are all in a very tight area:  from location 5,800,000 to 8,700,000bp.  Very few of them triangulate.

Sometimes the pile-up area has been documented. On chromosome 15, I have 281 segments in the 7 to 10cM range. They are at: 24,000,000 to 28,000,000 bp. This area partly overlaps a known pile-up area (20,100,000 to 25,200,000). But the known pile-up area is only partly the cause in my case. See 14 small pile-up areas found by Li et al (2014), listed at the ISOGG Wiki: http://www.isogg.org/wiki/Identical_by_descent These medium pile-up areas, and a few others in my experience, are characterized by a very tall pile-up of many segments about the same size in a narrow area just a little larger than the segments. The Li et al (2014) article refers to “regions where excess IBD is detected…” Virtually all of the segments I have noted above are IBS/IBC – they do NOT triangulate with the other segments.  A few segments in these regions do triangulate with known close relatives, and each other. I’ve kept those segments in maternal and paternal TGs, as appropriate, covering that area. After all, both my mother and father gave me those areas, and they in turn got them from their parents, etc.  It is very probable that these segments are IBD and come from a CA.

My experience is that these are areas with a lot of shared segments in the 7-10cM range that are in a tight area, usually just 10cM wide, and a very high proportion of these segments are IBS/IBC.  A few segments in these areas will be IBD, but they will tend to be larger than the 7-10cM segments.

My bottom line for these pile-ups: Unless you have a lot of free time, skip over these areas – particularly the shared segments under 10cM. Concentrate on triangulating any larger segments in these areas and then move on to other areas.

Large pile-ups – these are my favorites. Larger shared segments (over 10cM) that spread out and overlap each other over wider areas.  These segments tend to triangulate with each other, forming TGs on both sides.  I have some of these TGs which include over 50 shared segments.  Since the shared segments triangulate with each other, this is a good pile-up. These TGs are large because more people have these shared segments – probably because the Common Ancestors had large families in Colonial America, leaving us with many, many cousins. Another reason could be a more distant Common Ancestor, who would also leave us a large number of cousins.

In some cases we can use this observation to our advantage. I have a 2nd cousin, on his mother’s side, who is also an 8th cousin, on his father’s side. Our close Common Ancestor was an immigrant to the US in the mid-1800s, and I get relatively few Matches on the segments I share with him. However, on one segment, we have many Matches – it turns out our Common Ancestor is on his father’s side. The tip-off should have been the size of the TG (measured by the number of Matches).

Another observation about large pile-ups…. They will get larger. The number of folks taking an atDNA test is about doubling every 12 months. A consequence of this is that all of our TGs will also double in the next 12 months. So, if you have pile-ups now, they will about double by this time next year. Use these larger TGs to your advantage – work with the Matches to investigate place/time matches, if a Common Ancestor is not easily determined.

Summary

  1. In general, don’t work with shared segments below 5cM. Most are IBS/IBC – even if they appear to triangulate. We don’t have a good test below 5cM to indicate IBD.
  2. Watch for, and avoid, pile-ups in the 5-10cM range. These are characterized by many shared segments in the 5-10cM range in a very tight location- usually only 10 or 11cM wide. Move on to larger shared segments in other locations.
  3. Embrace the large pile-ups. They may from Common Ancestors with large families and/or more distant Common Ancestors. In either case, work with the Matches in these TGs as a Team to determine the Common Ancestor.

18 Segment-ology: Pile-ups by Jim Bartlett 20151007

49 thoughts on “Pile-ups”

  1. Jim,
    I’m trying to find the ancestors or siblings of my 2nd great grandfather. I have documented his wife’s ancestry in Scotland to 5th and some greater ggfs, but his (Joseph) family remains elusive.

    On MyHeritage, a 3rd cousin from this line and I have 1,448 shared matches, an insane number. A 1st cousin of mine has 1,445 shared matches with this same 3rd cousin. Using the MH triangulation tool, I have, so far, identified 206 people who triangulate on one segment of Chromosome 6; 10 people on a separate segment of Chr. 6, and 13 people on Chr. 4. One of these triangulates on both the separate segment of Chr. 6 and Chr. 4, though the Chr. 4 segment is under 7 cM.

    My 1st cousin’s matches show a similar trend; I just haven’t examined as many triangulations. I have looked at many, many pages of shared matches on MH, and I’m guessing that about 75% of each page shows a triangulation — insane.

    My first cousin and I share only about 25 shared matches on AncestryDNA. My feeling is the family I’m looking for is still in Europe — Joseph was born 1809 in England and migrated to Canada sometime before 1850. I have an extensive database of descendants from all children.

    If I understand your pileup commentary, the 206, Chr. 6 matches are all on a pileup region. At what point do the other 2 segments become pileup regions, too? What do I do with 1,448 shared matches with a 3rd cousin???

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    • Carolyn, This looks a lot like endogamy – perhaps a Jewish ancestor? You mention a 7cM Match – are the “pile ups” in the 7-10cM range, or 10 to 20cM range? or ?? And I’m assuming they all have the MyHeritage Triangulation Icon, which is often smaller than the shared DNA segment. In general, such a pile up indicates a very distant Common Ancestor, usually well beyond a genealogy horizon – I ignore these under about 12cM. For the most part I’ve stopped adding shared Matches below about 12cM to my spreadsheet. At the 2G gp level (your Target), you are looking for 4C back to his parents. At that level the average Match would share 35cM – give or take say 10cM . That is the range you want to find more about you Target. Hope this helps, Jim

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      • Jim,

        Thank you so much for your help.

        There is no indication of endogamy. Joseph was born 1809 in England 
        (Northumberland?), his wife was born 1821 in Berwickshire, Scotland;
        they married in Canada, and were Presbyterian. None of the 3 (me, 1C and
        3C) under discussion have any Jewish ethnicity. There was only one
        person who had a match on Chr 4 under 7 cM as well as a match on Chr. 6.
        14 of 38 on Chr 4 have largest segment 10+ cM; only 1 is over 30 cM.
        About 30 of 47 on one segment of Chr 6 have largest segment 10+ cM
        (largest unknown is 17.9 cM). I’m looking only at the triangulated icon.
        on MH.

        1C and John Groat share a segment of Chr 12. 1C and John have 1,445 (3
        less than I do) shared matches, and so far there are 93 shared matches
        on Chr 12. I assume that this must be another pileup region.

        On AncestryDNA: John and I share 25 shared matches; 1C and John have 23
        shared matches. There is one other descendant of Joseph on MH, Linda,
        3C1R. Linda and John (3C1R) share 12 cM/3 segments, and have 7 shared
        matches. Linda and I have no shared DNA. Linda and 1C (3C1R) share 13
        cm/2 segments and have 20 shared matches. On MH. John and Linda share
        39.1 cM, but I can’t see their DNA or shared matches. 1C and Linda share
        32.6 cM. John, 1C and Linda share 9.3 cM on Chr 12. 1C and John share 7
        segments, the largest 29.7 cM.

        John and 1C share 157.6 cM on 7 shared segments, largest 29.7 cM (on
        separate segment of Chr 6). John and I share 105 cM on 5 shared
        segments, largest 37.4 cM on Chr 4.

        Do you think it would be worthwhile to continue combing the shared match
        triangulations of 1C and John for matches on Chr 15 (they share 21.4 cM)
        and/or Chr 20 (they share 27.7 cM)?

        Otherwise, I’m completely befuddled. Thank you for taking time to read
        this mish/mash.

        Carolyn

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      • Carolyn, I, too, have lots of Matches – my Excel Spreadsheet has 28000 Segment in 372 Triangulated Groups (TGs), so the shear numbers don’t bother me. Shared DNA Segments under 20cM can be from very distant Ancestors. A thought – if your Ancestry is from UK – note that most of Ancestry’s customers and database is from America, which means the links to your family would necessarily be fairly distant to folks with US Ancestry.
        Each TG respresents 2 things: (1) a group of Matches who share a line of your Ancestry; and (2) a segment of DNA from an Ancestor. Actually the full TG would be from one Ancestor, but smaller segments may well be from a more distant Ancestor to that one.
        Per (1) – the objective is to look at the Trees of your Matches in a TG, and find the concensus Ancestor (or line – you might have a 3C, a 4C, a 6C, and an 8C all in one TG). So keep looking for Common Ancestors, until you are confident you’ve determined the line. Once you know the line, it’s easier to focus on that line while reviewing Trees and/or extending the Match Trees.
        FYI, you can look at the Shared cM Project at DNAPainter and get a good feel for the range of possibilities just by entering the shared cM amount.
        Jim

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  2. Hi Jim, I can’t believe I am just now seeing your post on this topic. I have sometimes heard the expression “sticky DNA” in reference to these larger segments. I have one for the Cates family of Henrico VA 1700 about 20 cM. Its over at wikitree on the profile Cate-16 But if you are using 10 cM as your base min then I have “more than one”. I am wondering if there are any developments as to how these are “monitored”. It would be great if some site was collecting these but I guess the database would be huge. The main reason that I switched to wikitree in 2013 was that they were posting atDNA on their profiles.

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    • Ray – Sticky and pile-up are two different concepts. Some piles – usually under 10cM are caused by false segments – often because of your own DNA at one location with a unique mix from your mom and dad so that almost anyone will match there – it starts with your own false segment. These are different for each person – identify the region for your self, but it won’t apply to others. Jim

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  3. They triangulate in the sense that most are in common matches with most of the matches in these clusters. None of them match every one of the in common matches in these clusters but they all match with most of them. I have not yet been able to find where any of them have branches of their trees in common.

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  4. Thank you for this post. I’ve been trying to figure out my “pile-up” region and I’m having a tough time of it. After I first got my DNA tested at FTDNA, I knew I had a large group of in-common matches. To give you an idea of the size, these matches are mostly 2nd-4th or 3rd-5th cousin range. They are definitely on my father’s side because they do not match my mother. When I compare numbers, I have more matches in common with most of these 2nd-5th cousin matches as I have for my two maternal aunts that tested! When autoclustering hit back in December, I found that I had two interconnected clusters with these matches. Doing a chromosome map for them 7 at a time, most match in the same range on my 6th chromosome and the rest match on the same range on my 14th chromosome. The ones that interconnect the clusters match on sections. I share around 35-70cm with these matches with the longest segments in the 12-23cm range. You can see my blog post about this when I first discovered the clusters back in December. You can ignore my conclusions I made in this post as I am now pretty confident they were wrong. At least you can see some screenshots to see what I’m talking about. https://matthewkmiller.blogspot.com/2018/12/autoclustering-large-groups.html

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    • Matt

      Have you Triangulated some of the segments in these groups? I have a similar “pile up” area, but the Matches don’t Triangulate – meaning they are false (IBC) segments. If they Triangulate, a large number in a tight group indicates to me a fairly distant Common Ancestor.

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  5. Hi Jim

    I think your blog is great – you give crystal clear explanations and teach such useful methods.

    I formed my first Triangulation Group using matches from MH, FTDNA and 23 sharing a segment of Chromosome 5 from 4-39 Mbp. Most were on MH which has a useful triangulation tool. I had a few questions to see if I was thinking about things the right way – apologies in advance for the length of this but the questions may be helpful to others as well.

    The TG has 176 matches (with a few duplicates) all at 12 Cm or more (except 1 at 7.5 Cm). Each matches and triangulates to a number of others in the TG. With the large number of matches, could this could be a “bad” pile-up area despite the high Cm level of the matches?

    Using the MH triangulation tool, I checked each MH match to see if they triangulated with both me and my mother (the segment is from one of her grandmothers). Would this help test whether the matches are IBD rather than IBC/IBS?

    Although 9 of the matches matched each of my mother and me (and many other matches too) on the segment, these 9 matches did not triangulate with both my mother and me using the MH triangulation tool. Could this be a personal pile-up area for those 9 matches (where they may just be an IBC/IBS match to each of us and a number of other TG members by chance) or is there another possible explanation? I share between 12-18 Cm with each of these 9 matches.

    Within the TG, there seem to be 3 distinct Sub Groups – SG1 has 63 matches from 4-8 Mbp to 15-18 Mbp (start and end points are fuzzy), SG2 has 46 matches from 22-25 Mbp to 33-36 Mbp (fuzziness again!) and SG3 has 67 matches that overlap both SG1 and (except in 4 cases) SG2 (with 10 extending beyond SG2 to 38-39 Mbp).

    Could SG1 and SG2 show possible crossover points within my segment (and thus SG1 and SG2 represent two different sets of Most Recent Common Ancestors to me) or rather show where there was a crossover for a MRCA of all of the matches in SG1 or SG2 that is more recent to them than the MRCAs I share with them (and thus their shared segment has a common endpoint before the end of my segment) or are both possible?

    There does seem to be a gap between the end of SG1 and the start of SG2 without a separately identifiable group that fills the gap alone. All matches in SG3 straddle the gap as well as overlapping with at least part of SG1 and (except for 4) with SG2. Would this support the idea that it does not show a crossover point within my segment or might the 4 matches whose segments end where SG2 starts hint at that being the crossover point?

    The longest matching segment (from 5-38 Mbp, or 43.5 Cm), which covers almost my entire segment, belongs to someone who has a tree where I can identify a possible MRCA (a couple I have seen identified as MRCAs in a number of other public trees but where I haven’t seen any documents to support the claim) but this would have been about 9 generations ago – so around 1750. Might a segment this size survive so long without crossover events?

    This segment comes from a line that was in colonial Virginia and then southwest Virginia, northeast Kentucky and south West Virginia, and that is well documented back to the 1780s. SG2 has a cluster of 17 matches in Finland (there were 4 additional Finns that did not triangulate with each of my mother and me despite matching both of us on the segment so I excluded them). The segment matches to this cluster over 12-16cm. Given the length of the match, is this likely to be a real match or a coincidence?

    If a real match, I can only surmise that either a Finn (or someone with Finnish ancestry) was in the right place in colonial Virginia or someone with a descendant in colonial Virginia (perhaps English, German, French or Swedish) also had a descendant who wound up in Finland.

    Apologies again for the length of this post – forming TGs is a great tool but once started they throw up a number of questions. I don’t expect you to have answers but any insight you can share would be appreciated!

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    • John, you’ve done a great job of Triangulation, and appear to have a good grasp of what is going on in this case. It sometimes helps to think of an Ancestor 10 generations back and how their DNA may have randomly passed down to all your living cousins in this TG. Each Match did not get the same segment you got – the each got some DNA segment that overlapped your segment – each in its own random way. And your cousins may be close or distant. When you see potential sub-TGs, there is almost certainly multiple MRCAs. All of our DNA continues to subdivide into smaller and smaller segments from ancestors further back – the problems are finding MRCAs that far back and sorting out IBD vs IBC segments.
      I have several TGs just as you describe. The more Matches in a TG, the more a) distant the MRCA is likely to be, and b) the larger the families along that ancestral line, and c) probably the more Matches from Colonial America (my one grandmother’s ancestors were immigrants from Scotland (CAMPBELL) and Germany (WEHRLE) in the 1860s and I’ve found only a few dozen Matches for that 1/4 of my DNA (compared to many thousands of Matches from the other 3/4 of my Ancestry.
      In my Chapter 1 of “Advanced Genetic Genealogy” I have an example of a large TG with a number of 6C, 7C and 8C on my HIGGINBOTHAM line. I’m sure some of the many Matches in that TG are 9C and 10C, but no one, so far, has actually worked out that genealogy – most of us have “borrowed” an incorrect line beyond 8C, so this is still a work in progress.
      With the sub-TGs, your closer cousins will overlap them, and the more distant cousins will go on separate lines which at some point merge at a husband/wife. Remember the DNA is random and won’t all be neat packages. However, in your body (and your mother’s), the crossover points are fixed – roughly 34 for each generation going back. So in your large TG, the start/end points are are crossovers from closer generations to you. This TG will have additional crossover points from more distant generations, but not one from each generation – the farther back you go, the more sparse the crossovers become as they skip generation more and more often (resulting in the “sticky” segments which you see from a distant Ancestor.
      It’s hard to give you an absolute yes/no to your questions, but I clearly think you are on the right track on all counts. Jim

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      • Thanks Jim – I appreciate your reply and it’s good to know that I’m on the right track. Thanks again for the great blog!

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    • I continue to view this as a two-part problem. One is the DNA part. All of your own DNA is real; it’s true; it’s IBD from a parent. If someone “matches” you on a segment, you have to decide if the shared segment is real (true, IBD) or not. I believe Triangulation of segments (multiple segments if you have any doubts) will cull out almost all false “matches”. If someone truly matches your DNA on any segment, then you probably got it from the same Common Ancestor. Now the other part of the problem is figuring out who the Common Ancestor is. With small segments (I don’t go below 7cM, so when I say small, I mean 7 to say 10 or 12cM), the Common Ancestor could range from a 3rd cousin (3C) to a 10C, or more distant. In other words many the Common Ancestor is within both of your genealogy Trees, and maybe it’s well beyond. Another way to look at it is: we certainly are not going to find the Common Ancestor with most of our Matches. For me the goal is to find a Common Ancestor with some of my Matches – enough to convince me that one ancestral line is correct for each TG. And this includes TGs which span pile-up areas.

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      • Jim I’ve followed your blog for years and much appreciate your contribution to our shared hobby. I remain however unclear what IBS segments really are. For example, say my mom has a 5cM segment that matches with another person A but has been “determined” to be IBS for both parties. But let’s say each party passes this 5 cM segment onto their child. Now I have that same 5cM segment matching the child B of my mother’s original match A. Is this segment now not IBD for for B and I?

        I can’t help but be sceptical about the term IBS. Can you explain IBS in more detail? How does a statistically large segment of size 5cM form to become IBS? Is there a visual explanation about how IBS segments are formed?

        My tree is very well researched and at least in the past several generations I have very few NPEs to cloud my research. But I am keen to breakdown walls in my 1500s, 1600s and 1700s research… and THAT simply won’t be done focusing on the low hanging fruit of 10+ cM segments.

        Your thoughts?

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      • Douglas, All of your DNA is true; all of your Match’s DNA is true. IBD, IBS, IBC, etc all refer to a “shared segment” which is determined by a computer looking at the DNA from both your parents. In the short run (almost always less than 15cM), the computer can “make up” a shared segment from parts of your DNA (and/or your Match’s DNA) that came from both parents – sometimes we say the algorithm zig-zags between the values both parents gave you. A shared segment (sometimes called an HIR – Half Identical Region) is a synthetic segment (only in a computer) which is not passed on at all. Hope this helps.

        Jim Bartlett – atDNA blog: http://www.segmentology.org

        >

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      • So Jim an IBS segment:

        1. is a conceptually constructed segment from two paired chromosomes rather than a real segment existing on a single chromosome?

        2. cannot be passed to a child because it is not a real physical segment?

        And the IBS match is caused by an algorithmic match of two such constructs… whose quantities exponentially increases with decreasing size?

        Therefore, the smaller the matching segment, the exponentially more likely it is actually a construct from paired chromosomes (ie both parents) rather than it is a real segment that has been passed down individually by one of your parents.

        Do I have it right?

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      • Douglas,

        I would request that we don’t define IBS (Identical By State) here. Even the scientists argue about its meaning and use. So, many genealogists (me included) like to use IBC (Identical By Chance). We need a term that means NOT IBD (Identical By Descent). IBD means we both got the same segment (on one chromosome) from the same Common Ancestor. This is the key concept in DNA matching. The general consensus is that all shared segments over 15cM are IBD. And so you have a segment one one chromosome that has the same long string of matching SNPs as your Match has on one chromosome. As you decrease the cM “length” of a computer generated shared segment, the probability increases that it is IBC (not IBD), and is not a segment on one chromosome for you and/or the “Match”. I don’t know if the shape of this cM vs % IBC is exponential, but it is at an increasing rate – definitely not a straight line. It’s roughly 50% in the 6-8cM range.

        The bottom line is an IBD shared segment, tells you the exact address of a segment on one of your chromosomes which you can use in DNA Painting and chromosome mapping. NB: the full segment you got from an Ancestor may well be larger than an IBD segment with one Match.

        Jim Bartlett – atDNA blog: http://www.segmentology.org

        >

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    • With about 2 million people having already taken an atDNA test, we are going to see our Triangulated Groups growing a lot. Each TG represents a DNA segment you got from a specific ancestral line. If there are a bunch of smaller shared segments in the TG – say 7-12cM – that is a very strong indication to me that they are from fairly distant ancestors – say 10-20 generations back – on average. So my focus would be on trying to find a Common Ancestor with the Matches with larger shared segments – they will tend to be closer cousins. Jim

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  8. Jim some follow up questions on your post: Summary 1: I have 2 siblings and my mother tested, as well as my father’s sister. If all three siblings and our mother, or in another case all 3 siblings and our aunt, have the exact same 4cM segment, what am I to make of this? It is obviously IBD for at least 1 or 2 generations… but could the original segment from my mother (or aunt) be IBS? So is the segment IBS or IBD? Summary 2: so is there an explanation as to why one would have several 9cM segment match pileups that are not IBD? Summary 3: Yes large cM segment matches are great… but I already know my full tree back several generations. It is the 7-15 generation range that I need to investigate… yet your post seems to imply don’t pay much attention to segments 12cM? Could you please post on how the tedious effort of triangularization will lead to 17th or even 16th Century genealogical value? Thanks.

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    • Jason; these overlapping segments are obviously overlapping. They may be true or not – that is the problem, we don’t have an easy way to tell. Using phased data is one way. But even if the overlapping segments are IBD, the highest probability is that they are from a very distant ancestor. You know of a close Common Ancestor, and they may be from that, but the segments could also come from a very distant ancestor. In general, we cannot rule out all distant ancestors. When we share a large segment – say 220cM – it’s fairly easy to rule in, or out, all the relatives who could share that amount of DNA. With 4cM segments the cadre of possibilities is just too much. We cannot just assume the 4cM comes from the closest Common Ancestor, or our favorite Common Ancestor, or a famous Common Ancestor, etc. Too many folks start with what they are trying to prove, and only look at data that supports that conclusion, without considering the other possibilities. Summary 2 – yes – sometimes our own DNA has a area on one chromosome that includes a unique combination of SNPs from both parents that permits a zig-zag IBC match. Many of us have found these under 10cM (very easy at GEDmatch) – in general this doesn’t happen above 10cM. Summary 3: Utilize combinations of MRCAs on the same line – either several, widely separated, Matches all agreeing on the same CA; or several, widely separated, Matches with MRCAs which are all on the same ancestral line for you – in other word a 2C, a 3C, a 5C and a 7C all on the same line – called walking the Ancestor back. It is not sufficient to just have the 7C and an MRCA (unless you can positively eliminate all the other possible MRCAs for that segment – a virtually impossible task) Jim

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  10. Thirteen generations is not possible for those of us with early Southern ancestors, where courthouse fires destroyed vital records in the 1800’s. For some of us, 1850 is an unsurmountable wall. Proving parents of poor Tennessee and Kentucky ancestors is often not possible. Then there’s the early 1900’s immigrant from a European country, where records burned during both world wars. Mid-1800’s immigrants from European countries that don’t have online records. The vast majority of my matches are distant, making DNA less helpful than it might be for others. Maybe a decade from now there will be enough tests to make connections more clear. I’d be thrilled if more people tested for matches instead of ethnicity matches, and added public trees with a minimum of four generations.

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  11. This article was very helpful. I was quickly able to identify that my matches near the end of chromosome 10 that I was wondering about fell into the large pile-up category. I started with 70 matches in that location and the list grew. By the time I added my mother’s results, there were well over 100 people who more or less all matched with one another. Several of our closest matches were over 20 and a few were over 30 cM. Through connections with some of these matches, I have now found that the roads all lead to County Mayo, Ireland before 1790. We have not yet found the common ancestor, but we are narrowing down the locations within Mayo and identifying a collection of surnames that pop-up frequently.

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    • Tetley, Thanks for the feedback. To my thinking, the only bad pile-ups are those with many segments under 10cM, in a tight group, which actually don’t match each other. The larger segments, particularly any over 15cM, are going to be IBD (from a Common Ancestor). If there are a lot of Matches in a TG, that generally means the Common Ancestor is further back in your ancestry; but some of the larger shared segments will be with closer cousins.

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  12. Thank you for this write-up. I have some lingering confusion that I hope you can help clear up for me. I just discovered about a dozen FTDNA matches for my father that are all tagged as Maternal. They all have roughly the same start and end locations (of about about 9.5 to 10.1 cM) on the same chromosome. I’ve run them all through the FTDNA Matrix tool and they all show as matching each other. Is this a medium pile-up or a TG? Sorry for being dense about it, but how are we to tell the difference between the two? If they do not match in the Matrix, then it is pile-up? A known 3rd cousin has this same segment, plus several others. How can I tell if this is an actual IBD segment and not an IBS segment that the 3C also just happens to have? I am new to looking for TGs, so I don’t want to start off on a wild goose chase! I’d like the first one to be legit. 😀

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    • Matt, Triangulation generally insures that shared segments are IBD. The FTDNA Matrix tool, is an InCommonWith tool that does not necessarily mean Triangulation, but when they overlap on the same chromosome, they are almost always Triangulated – so I think you are OK. I would not call 12 Matches a pileup. I have many TGs with 20-50 Matches; and new Matches pour in every day. A pile-up can occur in a TG – it’s just a lot of segments. A bad pile up is usually a lot of shared segments in the 7-9cM range all in a very tight area on a chromosome (maybe an area of only 10cM). Your best bet is to upload your raw DNA data to GEDmatch, where you can test your Matches’ segments against each other, to insure Triangulation. I lot of Matches in a TG, usually means the Common Ancestor of the TG is further back in history, but often some of the Matches will be closer cousins – like your 3rd cousin.

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  13. Echoing other replies here over a year after this post: this is extremely helpful information as are all your posts. Thank you for sharing and explaining your experiences. This seemed like it was too hard till I started reading and re-reading your blog.

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  14. I decided to use the GEDMatch chromosome browser to look at the top hundred or so my top X-chromosome matches. As you know, you can’t use X chromosomes to prove much if anything about distant genealogical relationships, but I wanted to look for patterns in what chromosomal regions are matched. I notice that there are definite patterns with strong pileups on the X chromosome, with several distinct regions, so it basically looks like 3 stripes on that chromosome,, and on a region in chromosome 12 in particular. Some chromosomes look like scattered noise, but not those.
    However – I know that Bayes and the birthday paradox may both play a role in these patterns showing up. I need to do more sophisticated analysis. Has anyone tried anything similar?

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    • Lois,
      I don’t know of such a study. IMO, each segment of our DNA came from a specific ancestor. All the DNA on a chromosome, including Chr X, came from a parent, who got it from their parent, etc. Somewhere going up our ancestry, that segment is a recombination from 2 ancestors, and it stops being an IBD segment. Drop back one generation and that ancestor is the last one to pass that entire segment to you. Without chromosome mapping we can’t usually tell which ancestor is the ultimate one. See my post on the porcupine chart. With Matching cousins, we can “walk each segment back”, and find this out. Somebody had to pass down each segment of Chr X. Yes, smaller segments may be from distant ancestors, so we need to look for the larger segments with closer cousins.

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  15. Thanks for your marvelous posting! I truly
    enjoyed reading it, you could be a great author.I will
    ensure that I bookmark your blog and will often come back down the road.

    I want to encourage yourself to continue your great job, have a nice day!

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  16. Hi Jim
    Do you have a sample spreadsheet that you use for chromosome painting and triangularization that you would be willing to share? I would like to start building a chromosome map and would appreciate starting with a tool that has been exercised and is set up fit for the purpose…

    Thanks for your blogs!
    Douglas

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    • Douglas, I’m working on several blog posts about spreadsheets. Mine has evolved, a lot, over the past 5 years. One caution: whatever you want to see in a spreadsheet, is what you have to enter and maintain – it’s literally a two-edged sword. I’ve got way too much data in mine. Way more than is needed for just Triangulation. But, I also use my spreadsheet as my master atDNA tool, diary and repository – it keeps everything in one place. I emailed you the format – watch for the blog post.

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  17. Very good article! I agree 100% that just calling the large pile-ups “pile-ups” is misleading because they are completely valid and the ones you WANT to focus on. John Abbott.

    Your success with triangulation is very encouraging to the rest of us –thanks!

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    • John, That’s exactly why I wrote the blog post. “Pile-ups” was turning into a bad name; and folks were beginning to believe that all pile-ups were bad news. As in many things, we need to look at the details and sort it out.
      Triangulation, I admit, is hard work. So I recommend just starting on some areas that are interesting to you. Gradually you’ll build out the framework of your segments on both sides.

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  18. Thanx again for very timely and informative article. I believe that Ancestry.com has thrown lots of babies out with the bath water. I can point to 4 matches quickly on my list with segments of 18.4 to 19.7 cM that disappeared with Ancestry’s first purge. The same 4 matches remain on the list of my 2nd cousin and I believe all of my segments are larger. I’m sure that there are many more. My confidence in their process is waning!

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    • Jeanette,

      Thanks for the encouragement. I’ve heard a number of stories of large segments which dropped out as well as Ancestry kit’s found at GEDmatch with large IBD segments that AncestryDNA didn’t report. That’s why I like GEDmatch so much – yes, I get more IBS segments, but with their comparison tools, it’s easy to cull them out.

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  19. Dear Jim, Thanks for sharing your work and your conclusions. I appreciate same and I have learned a lot from you these past few years. Linda McKee

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  20. Excellent work and very informative. A very nice 71st birthday present as well.

    In reference to Colonial Ancestry and your suggestion that it skews results; One issue with my tree is that almost all of my ancestors are Colonial American. Most of the rest are Canadian or are not identified yet. I believe that my newest immigrant ancestor arrived in 1829. Probably half of my ancestors were in the Americas pre 1700. A large number of my AncestryDNA matches show multiple, indicated by clickable arrows, up to 5 separate and different matches. What impact do you think might this sort of a tree might have. The overwhelming number of Ancestors go back to the 1600’s

    Also note, my tree is a 80,000 plus person “data mining tool” Questionable new data is allowed. But, errors are diligently searched for and when detected ruthlessly corrected. atDNA has been very useful in finding and correcting errors. I’ve bought atDNA kits for 8 people and work with a number of others who administer a similar number of kits.

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    • Sam – Thanks and Happy Birthday! It looks like you are well on your way with atDNA. I’m working on a blog post about endogamy. I personally believe it’s an overblown issue – some impact, but not much – but I’m crunching the numbers to give a fair look at it.

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      • Have you written anything on working with endogamous groups? Or can you recommend some reading that could help? The lines on my paternal side are very tangled. The group that I’m working with has about 60 kits on gedmatch. I match 88% of this group. The highest kit matches 93% of the group with the lowest matching 50%.

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      • Jeannette,
        I’ll start with the effect of one set of first cousins as ancestors and then extrapolate to endogamy – we’ll see how the numbers shake out.

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