As readers of this Segment-ology blog understand very well, DNA segments are passed down from our Ancestors, through a line of descent, to one of our parents, and from that parent to us. Well… you might say: I thought our parents passed down whole chromosomes to us – 23 of them to be exact. That’s correct – and each chromosome is made up of many segments. Even the segments are made up of segments. While a parent passes down 23 chromosomes to us, those 23 very large segments, are actually made up of about 57 segments from our grandparents; and 91 segments from our great grandparents – see Figure 3 in my post “Crossovers by Generation” here. From a theoretical perspective, these segments can start and end anywhere and there are infinite possibilities. But for each of us, all of our segments are fixed before we were born – they are very specific in our body.
For this post, I want to look at the life of a DNA segment which is represented by a Triangulated Group (TG). You will recall a Triangulated Group is a segment of our DNA identified by a Chromosome, Start Location, and End Location – it’s a specific part of a chromosome passed to us by one parent. This is real DNA in our body – a long string of millions of base pairs, usually represented by over a thousand unique SNPs (which are the markers which are actually measured in a DNA test). We can “see” this segment of our own DNA because of overlapping shared DNA segments with various Matches. Because multiple Matches share the same long string of SNPs with us, we understand that this DNA segment had to come from a Common Ancestor (CA) to us, and to each of our Matches in the TG. The science tells us that that’s the only way we would have gotten such long segments in common.
So, exactly where did this TG segment come from?
Which of our Ancestors first had that segment – the Most Distant Common Ancestor (MDCA)?
Which generation back?
How was it formed?
What did it originally look like?
How was it transformed into the TG segment we now see?
Are some of our Matches in a TG actually related further back than the MDCA?
Are some of our Matches in a TG actually closer cousins than the MDCA?
Read on – all these questions will be answered in The Life of a DNA Segment.
Some ground rules – we are not talking about small segments – let’s use at least a 15cM IBD segment. This is not hypothetical DNA in a mathematical simulation of many possibilities and variations with some distribution curve. We are going to look at a specific segment that was identified by a Triangulated Group – it came from one of our Ancestors. We start with a real DNA segment in our body.
Let’s just start with an example and work from there. Let’s say my TG-segment is:
Paternal Chr 04 from 20Mbp to 45Mbp. My TG ID would be 04C2 – meaning it’s on Chr 04, starting about 20-30Mbp, and on my father’s side . [You can review ID codes here].
Let’s say it first appeared in a 3xGreat grandparent – specifically (to be generic, I’ll use Ahnentafel numbers): my father’s [A2], father’s [A4], mother’s [A9], father’s [A18], mother [A37].
Figure 1 – My Paternal Chr 04 and TG 04C2 on that Paternal Chr 04.
In order for a segment to have “first appeared” in A37’s DNA, it must have been formed from (usually) two separate segments. Let’s say such a recombined segment was passed down by A37’s mother, A75. A75 had two Chr 04s (a paternal one from A150 and a maternal one from A151.
Figure 2 – A37’s Two Chr 04s.
Figure 3 – A75 crossovers and recombination of her Chr 04
In this diagram, A75 is going to pass to her daughter, A37, the yellow segments, with crossover points at 35 and 115 (two crossovers, creating 3 segments, are typical on Chr 04).
Figure 4 – Chr 04 passed from A75 to A37
In this diagram, the top Chr 04 shows the grandparent segments passed down to A37 from her mother A75. There is no clue to the future TG. In the second Chr 04, I’ve overlaid TG 04C2, so we can track what the future has in store. It now has a crossover in it that makes the area of 04C2 unique – DNA from two different people. As we shall see, this area of A75’s DNA was passed down to me (after all, we started with a TG that I had). Also, some part of this area was passed down to others who became Matches in TG 04C2 because their overlapping shared segments meant these Matches matched me and each other.
Let’s continue with the Life of DNA segment 04C2… A37 is going to recombine the above maternal Chr 04 with the Chr 04 she got from her father (probably with two crossovers again), and pass the recombined Chr 04 to her son A18.
Figure 5 – Chr 04 with A37 recombination and passing to A18
The top two are A37’s two Chr 04’s with crossover points at 45Mbp and 95Mbp, with yellow (and blue) highlighting showing the three segments that are recombined to make the bottom row: a single Chr 04 being passed to A18. Note that the 35Mbp crossover point exactly coincides with the end point of TG 04C2. When A37 passes a Chr 04 to her other children, some may get the part the includes the imbedded 04C2 segment, and some may not. NB: This is a critical generation. At some point on Chr 04, we must have a crossover point at 45Mbp, because my TG 04C2 had that crossover point – this is the generation where that crossover occurred.
Next A18 will pass a Chr 04 to his daughter A9.
Figure 6 – Chr 04 passed from A18 to A9
Again, the top two rows are A18’s two Chr 04’s, this time with crossovers at 60Mbp and 160Mbp, with highlighting showing the three segments that are recombined to make the bottom row: a single Chr 04 being passed to A9. Note that neither of the new crossover points had any bearing on my TG 04C2 – in fact that whole end of Chr 04 (1 to 60Mbp) passed intact to A9, as part of her paternal Chr 04. Next we’ll see A9 passing a Chr 04 to A4.
Figure 7 – Chr 04 passed from A9 to A4.
In this generation A9 has crossover points at 20Mbp and 120Mbp. Other crossover points may be at other points for other children of A9. However, the 20Mbp is needed here (or in some generation for me to wind up with TG 04C2 [Chr 04: 20 to 45Mbp] – a unique TG segment made up of segments from A150 and A151.
Figure 8 – Chr 04 passed from A4 to A2 and from A2 to me
I just added a few more crossovers here – they don’t really matter as far as TG 04C2 is concerned. The crossovers had to miss TG 04C2 in order for me to get that particular segment. Remember this is not hypothetical, we started with a real example of a TG. The crossovers could have been different at each generation, but somewhere along the line, before TG 04C2 got to me, it had to be boxed in at 20Mbp and 45Mbp in order for me to have that TG. Because I have multiple Matches in TG 04C2, each of them had to have similar stories – they rarely get exactly the same TG I got, they got different segments from A37, but there was overlap in the area 20-45Mbp. If the other Matches did segment triangulation around this same area on Chr 04, they might have gotten TGs like: 15-35 and 18 to 42 and 27 to 53, etc. See The Anatomy of a TG here.
There are many, many ways the life of TG 04C2 could have played out, but they all would have resulted in a unique segment at Chr 04: 20-45Mbp, because a bunch of my Matches had overlapping/triangulating segments with me at that location. We started with the fact of TG 04C2 and all the included Matches.
With this TG originating in A37, I can have 4th cousins (4C) on my Ancestral couple A36 & A37 (the DNA coming, in this case, from A37). I could also have closer cousins, maybe some of them sharing more than just TG 04C2 with me. Can I have more distant cousins among my TG 04C2 Matches? Sure, I may well have a 5C Match on A150 OR A151, depending on which path the DNA came down. NB: a 5C could only share a maximum of 20-35Mbp from A151 OR a maximum of 35-45Mbp from A150 (because that’s all I got). I could have a 6C or a 7C on Ancestors of A150 or A151 – depending on which path the DNA came down. NB: none of my Matches has to share the full 45Mbp with me, just enough to have overlapping matches. I cannot share a full TG (20-45) with a 5C – this segment did not exist back that far.
Let’s see if we answered some of the questions:
So, exactly where did this TG segment come from? From Ancestor A37 [this has to be determined by genealogy research among the Matches in the TG – Walking the Ancestors Back – A37 was assumed for the purposes of this example]
Which of our Ancestors first had that segment? All of the Ancestors in the line A2, A4, A9, A18, and A37 had that segment, but neither A74 or A75 carried that entire, unique segment – A37 was the first.
Which generation back? 5th
How was it formed? By recombination of A150 & A151 DNA by A75, when she passed a Chr 04 to her daughter A37 [however, a unique segment could be created many different ways].
What did it originally look like? Originally, in A37, TG 04C2 was just a part of Chr 04. Review Figure 4 – A37 had a very large segments from A150 and A151.
How was it transformed into the TG segment we now see? Subsequent crossover points nibbled away some of the DNA, until only TG 04C2 remained.
Are some of our Matches in a TG actually related further back? Yes, on smaller parts of TG 04C2 some Matches could be 5C or 6C or 7C, depending on the ancestry of the A150 and A151 segments, and depending if the shared segments are large enough to form Triangulation).
Are some of our Matches in a TG actually closer cousins? It is definitely possible. It’s these closer cousins that help us to Walk the Ancestor Back.
With reference to the real possibility that some of the Matches in TG 04C2 may be more distant cousins (beyond A37)… What if A37 was a Brick Wall? Those more distant cousins would be on A74 or A75 or A148 to A151. AncestryDNA shows ThruLines Common Ancestors back to 6C (the level of A148), so it is entirely reasonable to expect the DNA to “work for us” back that far. Look at the Trees of those distant cousins for a Common Ancestor, and build out their children and grandchildren. The DNA has done it’s part, the rest is a genealogy task to find the link.
One observation is that TG 04C2 (20-45Mbp) existed in all 5 generations from A37 down to me. That is the unique string of SNPs exists in the area 20 to 45Mbp in each of those Ancestors – a pretty sticky segment to think about. All of my Ancestors back to A37 had to have those SNPs, and each of the Matches in the TG had to also have some part of them (enough to create a Triangulation).
So there you have it – the birth and life of a TG – you know the general process of what to expect.
The above was a top-down description of the Life of DNA Segment 04C2. We could also build the story up – starting with my TG 04C2, and describing what that part of Chr 04 looked like at each generation going back, until the whole 04: 20-45Mpb segment didn’t exist anymore. In this example TG 04C2 would exist in A37, and it would not exist in A37’s parents: A75 and A76.
I think most of our TGs will be found to have Common Ancestors at the 4C to 7C level; and some of the Matches in the TGs will have CAs back to the 8C or 9C level (maybe some even more distant). I think this is promising for genetic genealogists – many of our Matches will have Common Ancestors within a genealogy timeframe. We just need to find them.
Can weird or unusual things happen? Perhaps. I really don’t know. I understand that some ancestor had to have the TG segment and pass it down to me, and parts of it to my Matches in the TG. I understand the TG segment is pretty unique. And because it’s also present in multiple, separated, Match cousins, I’m confident that it’s from only one Ancestor. Is it possible that all of the multiple Matches are also cousins on another Common Ancestor? Technically, yes – practically, no! Maybe among my 372 TGs I’ll find a few that throw me curve balls. But, in the main, I’m confident that virtually all of my TGs will sort out. With my fixed set of Ancestors, and fixed DNA segments – there is only one correct way to interconnect them – there is only one way the DNA from my Ancestors came down to me.
SUMMARY for the Life of a DNA Segment – I know the above description is hard to follow (it was hard to write!) But the summary is that each of our TG/Segments started in some Ancestor, and was passed down to us. It was passed down through one, specific line of descent to a parent to us. When the TG/Segment was first formed, it was part of a full Chromosome in that first Ancestor. Somewhere along it’s journey, the Start and End points of the TG were determined by new crossovers points that are added when a parent passes a new, recombined chromosome to a child. Portions of each TG/Segment were also passed down to our Matches. Our TGs are formed by our shared DNA segments with those Matches. The final TG/Segment is a fixed part of our DNA.
Before I leave this topic, I want to refer to my recent post: TG SUMMARY spreadsheet, here. This spreadsheet lists all TGs. It also shows known CAs for each TG – and imputes missing Ancestors. With enough genealogy work, this spreadsheet clearly shows me Walking The Ancestor Back in each TG. By comparing the Ancestor Ahnentafel numbers in adjacent TGs we can see where crossovers occurred. I believe, in this TG SUMMARY spreadsheet, we’ll find the crossovers for the Start and End of each TG – indicating how and where each TG was formed. This spreadsheet could be a KEY interlocking process that leads directly to a full Chromosome Map linked to Ancestors. I’ll write more in a separate post.
[05F] Segment-ology: The Life of a DNA Segment by Jim Bartlett 20220104 Happy New Year
Thank you for posting. I don’t completely comprehend, but I would like to make a distinction between an ancestral segment and a TG. Your post clearly shows that your ancestral segment 04C2 first “came together” in A37. To my way of thinking, 04C2 is an ancestral segment, because it traces back to the single, unique ancestor A37. Closer cousins may actually share the entire ancestral segment 04C2 to create a TG that spans the entire length of 04C2. In that case, the triangulation group is the ancestral group. On the other hand, it’s possible to get DNA matches on portions of 04C2 that come from ancestors even further back, namely A75, A150, and A151. For example, a cousin who matches on the A150 portion doesn’t have to match on the A151 portion. Would the mutually in-common matches on the A150 and A151 portions of 04C2 also be considered triangulation groups?
Andy, I like your thinking! Yes, we could call 04C2 an *ancestral* segment – it was first formed by A37 with the specific TG start and end points on a Chr 04. No previous Ancestor had that full segment. Be aware that A37 passed an entire Chr 04 to each of her children – the portion of Chr 04 which included at least 04C2 was to her child who was my ancestor, and to her other children who were the ancestors of my Matches in TG 04C2. The possibilities of how much of Chr 04 that A37 passed down to her children are all over the place, but for my Matches, it always included at least part of TG 04C2 (enough to overlap with others).
A150 and A151 are two different Ancestors. A150 provides only paternal chromosomes to A75, and A151 provides only maternal chromosomes to A75 – so there is no chance that any of their segments could form a TG. Maybe it would be possible with endogamy/pedigree collapse related segments, but I would think very improbable. At that point, I think we’d need to be thing two separate segments. And these two segments would be *re*combined by A75 when the passed them to A37, making the TG 04C2 segment for the first time. Please let me know if you see a flaw in this thinking. Jim
Jim, ancestors further back than A75 could also match on parts of 04C2, right? Any descendants of more distant ancestors who receive all or part of the A150 piece of 04C2 would match each other and also that piece of 04C2. Similarly, there could be descendants who receive all or part of the A151 piece of 04C2 who match each other, as well as the A151 piece of 04C2. The two smaller TG’s would be nested within the larger 04C2 TG, and they don’t have to match each other. I’m trying to understand what I see in my matches, where I think I see TG subgroups nested inside larger TG’s. 99% of my TG’s are from the paternal side (U.S., Georgia) because my mother was from the other side of the ocean. Any matches on her side are distant in time, and the segments are fewer and smaller.
Yes, the A150 seg and the A151 seg should make up (at the least) the ancestral TG seg. And remember what it took to form the TG segs. The component segs have to be smaller, and could very well be below the cM threshold for a match from the companies. Technically they would match, but often we don’t “see” both sides. When we do, it takes the Ancestor back another generation.
Does this mean I could tell which generation a segment was born by seeing at which degree cousin relationship their segment match to me is considerably smaller than the closer matches? I REALLY appreciate your work and thank you so sharing your insights!
It’s hard to come up with a hard rule, because the DNA is so random. The TG segment was born in a specific Ancestor, but it could have been passed down to the next generation (or more) in tact. I think the key is when more distant cousins are clearly from one side and always a smaller segment. It would take several data points, all in agreement, to be sure. Another method is to figure out the Chromosome Map for each generation. It’s trivial for the parent generation (23 chromosome segments per side), so the first one to tackle would be the grandparents (roughly 57 segments per side). In trying to figure that out, I got a sense of how much data that takes. Next is to break *some* (not all) of those segments apart into about 91 segments, per side, from the great grandparents. Again, it’s a lot of work – but along the way you’ll meet lots of cousins;>j. Jim
Thank you for posting that article. Really well written and helpful.
Geoff – I appreciate you kind feedback. I think we are going to find more ways to link our segments and Ancestors. Jim
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Very interesting, I like how you were able to trace the segment.
The TG segment in this post was a made up example. But I have traced many of my TG segments back by finding Common Ancestors with the Matches in the TG. This is genealogy work. I’ll explain more when I add to my DNA SUMMARY Spreadsheet post. In the figures in that post you can see the CAs I’ve determined in some TGs.