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

A Segment-ology CONCEPT and Thought Stimulator

Per Wikipedia: A haplotype is a group of alleles in an organism that are inherited together from a single parent, and a haplogroup is a group of similar haplotypes. Your atDNA segment from an Ancestor is likewise a group (or string) of alleles (SNPs) that is inherited from a single parent – an atDNA haplotype. The Match segments in a Triangulated Group (TG) have this same string of SNPs – they have matching shared DNA segments – and this group would then be a Haplogroup (Hg).

A Triangulated Group of segments would be a Haplogroup.

Wikipedia also notes that in human genetics, the haplogroups most commonly studied are Y-Chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups, each of which can be used to define genetic populations.

In exactly the same vein, a Triangulated Group (TG) defines a genetic population. It’s the population of descendants who carry the same segment of DNA passed down by an Ancestor. DNA test takers in this population have shared DNA segments with the same string of SNPs – they match each other!

An mtDNA Hg is often many thousands of years old (because the mtDNA rarely changes). A Y-DNA Hg is usually somewhat closer, and with a Big-Y test, is often found within a genealogical timeframe. My estimate is that an atDNA Hg (a TG) is usually 5-9 generations old – generally within a genealogical timeframe. We could argue that a TG Hg is a better tool than Y or mt. For me, it is a very good tool. In any case, each DNA Hg tool has strengths in genetic genealogy.

Note that the process of Triangulation culls out most, if not all, false shared segments. A few false Match segments (under 15cM) may slip in; but your own DNA, as the base in a TG, is true. If such an under-15cM Match is critical to you, you need to check for Triangulation with that Match segment as the base.

MUSING….

Dr. Tim Janzen – one of the earliest pioneers in atDNA (and my early mentor), has often advocated for a database of unique atDNA segments from our Ancestors. I used to think of this as a giant TG database and wonder how we would describe each TG. Now I think it would be an atDNA Haplogroup database, but still wonder how we would describe each Hg. Each segment would be unique to a specific Ancestor and would be on a specific chromosome (with start and end points). Note the chromosome could be maternal or paternal, depending on each Match’s ancestry. This segment would manifest itself in a TG, with shared segments from other descendant Matches. Each Match would likely have his or her own unique TG. These TGs taken together would represent an atDNA Hg from that Ancestor.

NB: if we can phase our data, we could actually record the SNP alleles (ACGTs) in each TG (or atDNA Hg)! Alternatively, by comparing raw DNA data among the Matches in a TG, we could probably determine the individual the SNPs. Remember your TG segment is the equivalent of phased DNA.

This post is about an atDNA Haplogroup. It’s a concept to think about. Your thoughts are welcome here.

[14B] Segment-ology: A Triangulated Group is an atDNA Haplogroup by Jim Bartlett 20230802

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

A Segment-ology TIDBIT

mtDNA is passed from a female Ancestor down the all-female line to each of us. A Triangulated Group (TG) DNA segment is passed down from an Ancestor to us. The concept of DNA being passed down a specific ancestral line – from an Ancestor to us – is the same. Such is also the case for Y-DNA – it is passed from an Ancestor down the all-male line to a man. In the case of mtDNA, the ancestral path is all females; in the case of Y-DNA, the ancestral path is all males; but in the case of atDNA, the ancestral path can zig-zag between male and female Ancestors. Any of our Ancestors could pass an atDNA segment down to us.

The point is the TG segment is found only on one specific ancestral line (like the mt or Y line). However, it is still a genealogy task to figure out which line. As we “walk the segment back” from our own DNA back up our ancestry, there are only two options at each generation. If we know a TG segment is on our maternal side, the next generation back must be one of the maternal grandparents – and so on.

Just as we use mtDNA or Y-DNA, looking for a someone who shares that same DNA with us, to find our Common Ancestor; so, too, we understand that our atDNA Matches in a TG (thus sharing that same atDNA with us) will have a Common Ancestor with us.  

This is just another way to think about our DNA segments – they are just as focused as the mt or Y on *one* ancestral line.

[22BX] Segment-ology: Triangulated Group Segments Are Like mtDNA TIDBIT by Jim Bartlett 20230728

A Segment-ology TIDBIT

Here is the set up for my BROWN story, without dragging the reader through the whole back story. This line includes most of the descendants in the BROWN Y-DNA Project Group-40.

I’m searching for the children of Wilson BROWN (he probably had 10 children, only two daughters are known). This is my Tree at AncestryDNA. I expected ThruLInes to find some Matches… Nada. I had Matches from Keziah (and her husband Elliott BAKER) on down. I had none from Wilson – not too surprising because no one has any Trees for Wilson (except for daughter Keziah). I expected some 6C Matches from James, because I know they are out there – but… nothing.

So I used my “Search on a Surname” process [here] – I searched for the BROWN surname, and checked each Match’s Tree for likely families. One family that quickly became the standout was the family of Thomas BROWN 1773 married Nancy LITTON. I was getting a lot of “hits” on that family. So, I looked them up at Ancestry – there are 2,668 Trees for that line! Almost everyone who shows his parents, has John/James BROWN b 1731 MD; married 1755 Plymouth, MA; d VA & Sarah LITTLE b 1737 VA; d 1779 VA.

Two key points about Thomas BROWN 1773:

1. I have found over 70 Matches who descend from Thomas BROWN b 1773 (shared DNA segments from 10 to 30cM). These are spread over virtually all of his children.

2. Two descendants of Thomas BROWN 1773 – through different children – have taken a Y-DNA test and are in BROWN Group-40. So, Thomas BROWN 1773 is BROWN Group-40. No one else in Group-40 has claimed descent from his father, John BROWN 1731.

I have concluded that Thomas BROWN 1773 must be a son of Wilson BROWN and so I added him (and his children) to my Tree.  I stand alone in doing so…

I waited over a month for Ancestry’s ThruLines to show me the 70 Matches I had found – nada. Disappointing… Ancestry clearly had Thomas BROWN 1773 locked onto John BROWN 1731. I’ve written at least 10 blog posts about the power and usefulness of ThruLInes – search for links to them in the Segmentology Outline [here]. One post is about ThruLines X-Ray vision looking into Private Trees…

So, I decided: maybe Ancestry is correct! Maybe if I accepted their version, ThruLInes would report some of my DNA Matches as cousins. So, I changed my Ancestor Keziah BROWN from the daughter of Wilson BROWN to the wife of Thomas BROWN 1773 (so the two of them looked like the parents of my ancestor, John Brown BAKER – almost like Thomas had an affair with Keziah.)

The next day Ancestry listed 31 new ThruLines Matches (spread from 6 to 30cM) – all descending as half-cousins from Thomas BROWN 1773 – WOW.  All of these were new to me. 2xWOW! Near the top of the list was a Match with a Tree with only 2 parents, and 3 grandparents – ThruLines built the Tree back to Thomas 1773. I have built a lot of Quick&Dirty Trees in my BROWN searches, but I would not have tried that one. 3xWOW! And another Match had a Private (but searchable) Tree. I’d never have found that one. 4xWOW!

The fact that I adopted the on-line version of BROWN Tree does not detract from my goal: find more DNA Match cousins from Thomas BROWN 1773. And ThruLines delivered.! The Matches share DNA with me (no matter how the Tree is drawn).

I still need to put all of these in a spreadsheet; make sure they are reasonable; figure out the averages and see how they compare to the Shared cM Project. And I’ll wait a few more days – fully expecting another tranche in the next day or two.

BOTTOM LINE:

This method will sure save a LOT of scrolling through all the thousands of 8-9cM Matches for BROWN Matches (it took me over a Month of steady focus to just get through the 10cM BROWN Matches). And it will find cousins with Private Trees and cousins with very small Trees that don’t have BROWN in them!

[22BW] Segment-ology: Getting ThruLines to Work for Me TIDBIT by Jim BARTLETT 20230707

A Segment-ology TIDBIT

I contend that segment Triangulation will identify most of the false shared DNA segments reported from your DNA test. This includes a Match with one segment which is false; as well as a Match with multiple segments, some of which are false. I have Triangulated DNA segments at FTDNA, 23andMe, MyHeritage, and GEDmatch, and found many false segments (segments which did not Triangulate with other overlapping segments). In almost all cases these false segments are under 15cm. I cannot guarantee that all the false segments can be identified this way, but I am confident that most can. Triangulation is a time-consuming process – starting with a download of all your segments from one company at a time; sorting them by Chr and Start, and then working down the list to see which ones Triangulate. I did a blogpost using MyHeritage as an example for segment Triangulation: https://segmentology.org/2020/12/29/triangulating-your-genome/

Warnings: this takes weeks; there are some Triangulations that are difficult – just skip over these; a few might slip through the cracks; there may be some bare spots in your DNA.

Special Note: Although a Match’s segment(s) may be false, that does not mean the Match is not a cousin. This looks like a double negative, so let’s phrase it this way: a Match may be a cousin and not share any DNA segment with you, or they may share a false segment with you. In fact, about half of your true fourth cousins (4C) will not share a DNA segment with you.

Recently, at the Genetic Genealogy Tips & Techniques facebook group, there was a post looking for ways to identify Matches at MyHeritage which are random junk. Segment Triangulation would identify a lot of false segments. However, at MyHeritage, it might be efficient to just download all segments, focus on those below 15cM (where most false segments would be) and work down the list to see which Match segments don’t have a TG Icon. Still a lot of work… If anyone tries this, please post about your experience – we can learn from each other. 

[22BV] Segment-ology: Identifying False Shared DNA Segments TIDBIT by Jim Bartlett 20230618

A Segment-ology TIDBIT

I have long been a proponent of segment Triangulation (and Triangulated Groups (TGs)) and also Shared Match Clusters. Both of these are powerful tools. Both TGs and Clusters group your DNA Matches who share the same Common Ancestor (CA) with you.

But this is just a means to an end. By themselves these groups (TGs and Clusters) do not magically name an Ancestor, they point to a specific, but unnamed, Ancestor. They are just groups of Matches. We must also use genealogy!

By analyzing the Trees of Matches in a TG or Cluster, we can often find a consensus Ancestor. This Ancestor may be a known ancestor, and the Matches’ Trees may provide additional information for our research. Alternatively, this Ancestor may be a new Ancestor for us – a bio-Ancestor, a Brick Wall Ancestor, or even a “floating” Ancestor (unknown connection to our Tree). Or perhaps a fluke, a coincidence, a curve ball from our DNA Matches.  Although a fluke is possible, as you research this new “ancestor” more, it either becomes more and more probable as your Ancestor, or less and less likely. In my experience, the evidence usually starts to mount . In only one instance for me did it pretty quickly fall flat (and in that case, I found a “secondary” consensus Ancestor in the group which worked out). As usual, treat this “consensus Ancestor” as a good clue.

Another way to frame this is: TGs and Clusters are good tools – more genealogy work is needed to make them useful – to find out more about your Tree.

The point of this TIDBIT, is that forming TGs and Clusters are good processes, but they are only a means to an end. IMO, they are definitely a step in the right direction, but the research journey is not over with that step. We need to take the next, genealogy, steps of analyzing the groups to find the CA and then integrating that information into our own genealogy.

BOTTOM LINE: TGs and Clusters are a good step – analyzing these groups is an essential next step.

[22BU] Segment-ology: A Means To An End TIDBIT by Jim Bartlett 20230611

I have just updated the Outline of Segmentology. This is located in the black bar in the header of every page – just click it. The posts over the years have jumped around; the Outline tries to put the titles in some order – sort of like a Table of Contents. Each one is hyperlinked to the blogpost.  This Outline provides an easy way to scan through the topics and blogposts and navigate to the ones that interest you. Don’t forget to read the comments and questions for each blogpost – they often contain additional information.

[00] Segment-ology: Segmentology Outline Updated by Jim Bartlett 20230604

A Segment-ology TIDBIT

Subtitle: Filter Your AncestryDNA Match List

Bottom Line Up Front: Filter your AncestryDNA Match list by cMs when your objective is a distant Ancestor. Also use Side and Surname filters to further reduce your Match list for review.

Most of us use the Shared cM Project to look up the Shared cMs of a DNA Match to see the possibilities of our relationship. We’ve learned that 3,500cM means a parent/child relationship; 2600 cM is a sibling; 1750cM is a small group of close relatives; 880cM is often a 1st cousin (1C), or other close relatives. And we understand that as the shared cMs get smaller, the range of possible relationships expands.

But we can use the Shared cM Project in the opposite way – what is the probable shared cM for a given relationship? We see narrower ranges for close relationships, and fairly wide ranges for more distant relationships. However, even though the range may be relatively wide for distant relationships, the average cM does shift down as the relationship gets more distant. For example, a 3C relationship would not have more than 234cM; a 7C would not share more than 57cM (and the average for a 7C is 14cM).

Suppose we are focused on a particular Ancestor in our Tree. Our Target could be a brick-wall Ancestor; or a potential NPE (Not the Parent Expected) Ancestor; or a suspicious Ancestor; a known Ancestor for whom we’d like to find a new cousin who has some in-depth knowledge of that family; etc. We can use the Shared cM Project to narrow down our search! This works for close relationships as well as distant ones – my focus here is on the distant ones, but the process is what is important.

A good site to use the Shared cM Project is at DNAPainter: https://dnapainter.com/tools/sharedcmv4 – this site provides a number of interactive tools along with the basic Shared cM chart. For example, a 5C relationship (back to a 4xG grandparent – we have 64 of them) would average 25cM – and 50% of them would be in the under-20cM range. In this case we might want to start with the 50% in the over-20cM range. But for a 6C relationship (for 128 of our Ancestors) the average is 18cM and 70% of the Matches will be in the under 20cM range. Each of us gets to pick our own objectives and projects to pursue. If yours is like many of mine and beyond the 5C range, working within a range of cMs may be helpful.

Here’s how. At AncestryDNA, open up your list of Matches, and look at the row of “Filtered by:” tabs – we can use many of these in combination.

For this example I’m going to use the Shared DNA tab to select a range of cMs, AND the Groups tab to select my Maternal side.

In the project I’m working on, I’ve already looked at the Matches down to 16cM. By selecting a range for the next search, I also speed up the time for AncestryDNA to produce my filtered list of DNA Matches. In this case I’ll be looking at 11cM to 15cM. Remember to Click on Apply!

I’ll also filter my list by using the Groups tab to restrict the list to only Maternal-side DNA Matches:

In this case, I’ve chosen the Maternal side (remember to click on Apply). This will filter out a lot of Matches that don’t really apply to my Target Ancestor [I can later select the “Unassigned” Group to check those, too.]  Note that you can ALSO select several categories under Custom groups – such as New Matches (especially helpful if you want to revisit this project at a later date to check on new Matches), and/or any of your “dotted” Match categories (not much help in my current project because I’ve already visited – and dotted – all of those Matches I could determine, and I’m looking for new ones now in the under-20cM range.) In some projects, this “dotted” filter may be valuable.

And there is one more filter I often use. For my current project, I am looking for my DNA Matches who have a BROWN Ancestor. So, I click on the “Search” tab, which brings down a row of options. I type BROWN into the “Surname in matches’ trees” search box:

You can decide to check the “Include similar surnames” box, or not. For this project, I got plenty of results with just BROWN, and, if I wanted to, I could go back and try BROWNE or BRAUN, etc. I’ve had mixed results with the “Birth location in matches’ trees” filter box – sometimes the result is either the surname or the location, and I’d be wanting both. I did have very good results on a project with HIGGINBOTHAM surname and Amherst County, Virginia location. You might need to try some combinations to see which works the best for your project. Remember to click on the blue, highlighted Search box to include this filter.

This process of filtering is a powerful way to shorten your list of DNA Matches, tailoring them to your project goals. The addition of a cM range has helped me focus on more distant Ancestors and to speed up the AncestryDNA listing algorithm.

BOTTOM LINE: When you are searching for DNA Matches, think about the best way to combine the filters (including a cM range) and search parameters at AncestryDNA.

[22BT] Segment-ology: Using the Shared cM Project in Reverse TIDBIT by Jim Bartlett 20230601

A Segment-ology TIDBIT

Bottom Line: We cannot tell from a single Match; but there are at least three ways to figure it out.

SETUP: One cousin, or even several cousins, who share a Most Recent Common Ancestor (MRCA) with us. The “Common Ancestor” is really a Common Ancestor “Couple” – usually a husband and wife. You descend from one child of this couple, and the matching Cousins descend from one or more of their other children. The question comes up, which parent passed down the DNA segment to us? From this data, we cannot tell which parent passed down the DNA. All we know is that one or the other parent passed it down. I know of three ways to figure this out – maybe you know of additional ways…

1. Grouping *to* the parents’ parents. One DNA Match-cousin back to the MRCA won’t do it. We need a group of Matches – at least one back to the MRCA, and other Matches – often with smaller shared segments – that go back another generation. The group can be formed via Clustering (grouping Shared Matches); or segment Triangulation (or DNA Painter). The goal is to find a Match in the group who is a cousin at least one generation farther back than the MRCA. This will almost always tell you which side of the MRCA the DNA came from. Note: there is a very low chance, this might not work, but finding more than one more distantly related cousin on this line adds insurance. It is best to do this with segment Triangulation which represents a single DNA segment going back, but Clustering works, too. As you find more and more Matches with the MRCA, eventually (with smaller cM segments) they will break into two groups, one for each parent in the MRCA. Each of these groups will be based on a different DNA segment (a different subset of a Triangulated segment, involving smaller shared segments).

Note this is really a subset of Chromosome Mapping, and/or Walking the Ancestors Back.

2. Different partners. If one of the parents had children with a different partner (married or not), and the other Matches descend from this other partner, then you know the shared DNA segment had to come from the Ancestor who had multiple partners – the same DNA could not have come from different partners.

3. Differing ethnicities. If the two parents in an MRCA have very different ethnicities overall (or you can tell the ethnicity is different for this specific shared DNA segment (usually a TG segment), then a review of the Matches’ ethnicities might indicate which one passed down the DNA.

As with many things in genetic genealogy, the DNA may throw you a curve ball. I’m old enough to remember the Mickey Mouse Club on TV – and the “anything can happen day”. As you continue on your genetic genealogy journey, the evidence will mount. It should all point to the same results – which match your unique Ancestors and DNA segments.

Feel free to use this blogpost as a way to answer this recurring question on-line.

Amended slightly to call out the different partners as one of three ways.

If you have a favorite method of figuring this out – please post in the comments.

[22BS] Segment-ology: Does the DNA Come from the MRCA Father or Mother? TIDBIT by Jim Bartlett 20230515