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