Well, it depends… Your chromosomes are very large segments, which are not very old at all. On the other hand, I have some small DNA segments from Neanderthal Ancestors – pretty old. In general, the smaller the segment, the older it is. But let’s think about this for a moment.
This discussion will be about your DNA segments – large segments from close relatives to ever smaller segments from more and more distant relatives. They are all part of the DNA you inherited from your Ancestors. Segments are formed at the moment of conception – when sperm meets egg – about nine months before you were born. They don’t change until you pass them on – after recombination and new crossovers – to the next generation. So our unit of “age” measurement is a generation.
So, let’s start with the largest “segments” – your 44 autosomes passed to you by your parents. How old are these 44 chrsomosomes? Well, they are 0 generations old. You are the first person to ever have each of these specific – full chromosome – segments.*
Then let’s look at your grandparent segments that make up your chromosomes. On average you have 22 chromosomes, subdivided by 34 crossovers, for 56 grandparent segments per Side. These were each part of full, new, chromosomes passed from your grandparents to your parents; and then, one generation later, passed to you by a parent – they are 1 generation old. Again, due to random recombination for every child, you are the first person to ever have these specific segments.*
Similarly, your great grandparents, passed new chromosomes to your grandparents, who passed segments to your parents who passed segments to you, which would be unique and 2 generations old.*
You get the picture. The unique segments in each of your Ancestors are recombined into new segments and passed down – generation after generation. Your segments are “imbedded” in the chromosomes and large segments they passed down. And knowing the genealogy of each segment, we can count the generations to find their age – always one less than the number of Ancestor generations back.*
* So what’s with that pesky asterisk? In short, “sticky” segments. Some segments are passed down intact – they are exactly the same segment in an Ancestor and their child (who is also your Ancestor) – they were not subjected to a recombination crossover. More likely than not, one of your smaller chromosomes (Chr 18 to 22) was passed from a parent to you intact. So, in that particular case, it’s age is 1 generation (not 0 generations like all the other chromosomes). And this happens to some of the other segments passed down at each generation. Above we noted that you got about 56 grandparent segments from one parent. When you pass these to your children, recombination will create about 34 new crossovers. In general, they will be subdivisions of 34 of the 56 grandparent segments passed down to you – leaving 22 grandparent segments intact. You only pass half of your DNA to each child, but that still includes about 11 grandparent segments which are now 2 generations old!
It gets complicated real quick!
This is one of the reasons that as segments get smaller, the range of possible relationships increases. A given segment may have persisted for several generations, or not.
Chromosome Mapping of segments with MRCAs let’s us figure this out. Even if our Map is not complete, at least in some areas of our chromosomes can be figured out. Someday… it will be interesting to try to determine a Shared cM Chart which figures in the age of the segment. I’ll bet the ranges would be somewhat smaller…
[O5H] Segment-ology: How Old Are Your Segments? by Jim Bartlett 20251218
segment-ology 
Hello Jim , can you help me with a doubt?
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Jim , i have a dna match in myheritage of 13 cM , I match with this person and also with their cousins, third, fourth, and fifth cousins. Are these types of matches the strongest indicators that the match is actually IBD?
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kalque; YES! For several reasons. 1. 13cM is very close to 15cM where almost all Matches are found to be IBD (true). Very few 13cM Matches will be IBS (false) – AND with todays algorithms, the companies are weeding out more and more IBS Matches. 2. The fact that you, independently, match others is a strong indication of IBD. 3. Now, check for Triangulation at MyHeritage (indicated by a symbol next to Matches on your Match list there. All of the Triangulated Match segments should be overlapping each other on the same chromosome. Triangulation = IBD also. [I’m deleting your other two duplicative comments – new member comments are not automatically posted until I can review them; but from now on yours should post without my moderation) Jim
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Hello Jim,
I’m new to your blog and am learning a lot – thank you. I have a question that I wondered if I could get your opinion on please. I have a DNA relative that I’ve matched with on both My Heritage and Ancestry. We share 115cM across three segments. One segment is 96cM, with the other two segments being 10cM and 9cM. I’ve mapped our respective family trees and am very confident that our relationship can be no closer than 3rd cousin twice removed, and possibly further distant than that. My question is, how common is it for such a large shared segment to be inherited from a distant ancestor, and how many generations can we reliably expect that such a large segment could make it through unbroken to the present generation?
Thanks in advance.
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Anna; My advice is to use DNA Painter at: https://dnapainter.com/tools/sharedcmv4 Plug in the total 115cM and look at all the possibilities – I don’t have any better insight than the thousands of folks who have reported their experience. At this site 3C2R shows up as a 4% probability – possible, but on the fringes. If I had to bet, I’d say there is a closer, perhaps hidden or previously unknown relationship. I’d certainly be looking at the Tree of this Match and the where/when of their closer Ancestors. I’d also do the LEEDS method (try Ancestry first) to find four groups of Matches – each group being from a different grandparent. Then look at your top Shared Matches of the 1115cM Match to see which grandparent line they are from. Then look down the list of Shared Matches in your grandparent group to see if you can split that group into two groups (one for each of the great grandparents) The goal is to find a solid Cluster (Group) of Shared Matches, not too far back, which includes your 115cM Match. Set your genealogy aside and see if you can determine *their* Ancestral Tree. If they can all coalesce around an Ancestor, there is a high probability that that’s your Ancestor too. Good luck – and please give us some feedback how it goes. Jim PS: if that doesn’t work, then use MyHeritage to find other Matches with Triangulated Segments around the 96cM segment….
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Thanks for the reply Jim. Much appreciated. The 96cM segment really has me flummoxed, as it is the single largest segment I share with any DNA relative, although the two remaining segments are relatively tiny in comparison.
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Anne – Thnaks for you kind feedback. I’d really set aside your genealogy and try to find a Common Ancestor among the Shared Matches you have with the 115cM Match. Jim
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lovely explanation.
thank you.
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So 56 grandparent segments turn to 22 segments at 2 generations. Each generation thereafter would result in 22/56 as many segments. So 3 generations = an average of 8.64 segments, 4 gens = 3.4; 5 gens = 1.33 gens; 6 gens = 0.52; 7 gens = 0.21; 8 gens = 0.08; 9 gens = 0.03; 10 gens = 0.01
Then is this saying that the chance of any of those original grandparents being fully passed down is about 1 in 100?
If so, then it means that the chance of any grandparent-sized segments being passed down 8 more generations is very small, and that the likelihood is that these segments are less than 10 generations old.
Wouldn’t the same logic hold for smaller great-grandparent segments, that they would likely be less than 11 generations old?
And if you carry that back to 10th great-grand parent segments, which now would be quite small, that they would likely be less than 20 generations old?
Which seems to indicate to me that it is very unlikely to have any very old identifiable segments of 20 generations or older.
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Louis – I should clarify: The 56 grandparent segments turn to 22 segments that are 2 generations old plus 34 subdivided/recombined segments (now 68 of them) are 1 generation old. The point is that at first glance, the age is the Ancestors minus 1, but some segments are passed intact, so they are older (they originated further back. This simple math breaks down going back – just like the math that says 1C share 880cM, 2C 220cM, 3C 55cM; 4C 14cM; 5C about 3cM which we wouldn’t see. Maybe this is why AncestryDNA appears to limit all of our Matches to a 4C relationship at the max.
When we look at adjacent TG segments on one chromosome (for which we have consensus MRCAs), we can see the crossover points. Jim
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This does indeed get complicated. The most confusing thing to me is what to actually consider as a TG. It seems this needs to be married with a generational criteria. So if you have about 380 TG’s identified these may include segments inherited from a combination of G, GG, & GGG grandparents. If you restrict this to the more recent ancestors the TG’s combine and reduce the quantity. Or, if you seek to “walk-the-segments-back” to further generations at least some of the TG’s separate into multiple smaller TG’s.
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Yes it is complex. A lot depends on your objectives. One of mine is to walk my segments back. I’ve been trying to walk them back to the earliest Ancestor (MDCA – the Most Distant CA) who had that segment. At some point, every one of my 372 TG Segments goes back to an Ancestor, but then is not found (as a full segment) in either of that Ancestor’s parents. This means an Ancestor recombined his/her two chromosomes and passed down (for the first time) a new segment made up parts from both his/her parent’s DNA.Parts of the original TG segment came from the two different Ancestors. Like an archeological dig, the deeper you dig, the more you find from an earlier timeframe. I’m trying to discover how far back my 372 TG segments go. It’s relatively easy to determine which parent, even which grandparent, each TG segment passed through on its way to me. With each generation going back it gets harder. When part of a TG goes back another generation, I have to figure out if the whole TG went back or if the “part” TG is really a smaller segment going back on one parent. I think the best way is to try to determine all the grandparent segments, then the Great grandparent segments, etc. Jim
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Louis,
Since I believe a lot of our DNA segments are sticky (and my math indicates they must be), I think a lot of them are older. Every sticky generation is another generation of “age”. When I tag a TG segment with a CA, I often see other Matches with the same TG who share a more distant CA. In other words, Matches with the same TG segment line up on an ancestral line. Jim
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Well Jim, you’ve determined (and it’s amazing that you have!) how many generations each of your 372 segments go back. And 372 segments over both your father and mother’s sides would average to about 16 cM each.It might be useful then, if you could determine the number you have, and minimum, maximum and average generations that your segments are, in groups maybe of < 10 cM, 10 cM to 20 cM, 20 to 30 cM, and > 30 cM.
That might show a lot with respect to how old segments are of various lengths.
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Uhm io ho sul chromosoma16 questi piccoli segmento triangolato di 7,1-9cM e il chromosoma16 non è piccolo ho visto che ai detto dal chromosoma 18 al 22
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