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 
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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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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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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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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