For a DNA segment of 20cms, how far back is reasonable for a most recent common ancestor? [closed]

Question

Hi,

I've been working on trying to identify the MCRA for a 20 cm segment on chromosome 7..  This seems to be a fairly common matching point, but no one has been able to find a common ancestor, which makes me think that it is further back.

-In one match, I found a match 6 generations back, but the final connection is unproven.

-In another, I found a common ancestor with both, but its 14 generations back [[Clarke-4354|Henry Clarke]].

Is that reasonable?  On this tree, Henry Clarke shows up as an ancestor at least 3 times, and there are plenty of branches that may not go far enough back to see the common ancestry.

I'm just wondering what my next step would be. If that is a reasonable time frame, then I guess I would continue to look for additional matches.  If not, I'd likely exclude those as the potential common links and focus on the other less developed branches in hopes of finding  a more recent connection.  

Thanks for any thoughts.

Comments

What is your Cole connection, I have Scottish matches with Cole Also.

Corneleous Cole.

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Originally, Kohl from Germany.

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I don't think that you will find 20 cm at 14 generations.

The most interesting that I have found at 14 (and there is only one possible source, and these are all unique segments - I pitch the rest - up to 6 plus 3 in chrom 1 and 6 plus 3 in chrom 12.  They occur at exactly the point where my Baltic Noblity and some Russians I am close to connecting have valid matches at 7 or greater.

You can see my comments elsewhere on retention.  I usually can get 7th to 8th cousin on 20 cms - but of course that means that I only have so many families with whom to match.  Fortunately some are useful

Answer 1

A single 20 cM segment is tough to pinpoint exactly.

Currently - the only known way to isolate the origin of a segment of this size is through triangulation, in which you make the case that three or more independent descendants of common ancestors inherited the segment.

For example - my Dad and two of his second cousins - 1 X removed (Jane and Darlene) have A.B. Jones (1836-1902) and Mary Coates (1839-1893) as their recent common ancestors.  I feel very confident that these are their only common ancestors going back five generations.

They have a triangulated segment on chromosome 13:

	R. Jones		*jane	13	47511636	73281941	16.0
	*Darlene		*jane	13	52131377	92330864	30.1
	R. Jones		*Darlene	13	52207434	74159637	15.3

 

So - I have reasonable case that the each inherited this segment from A.B. and Mary.

 

Still though - given that the segment is between 15-30 cM in length, I do need to consider the possibility that they could have another common ancestor from further back in their respective trees.  While I have no evidence of this, they each have other Colonial American ancestors, so I can't rule out the possibility that they might have another ancestor 8-10 generations back for example.

 

A triangulated segment and the consideration of where the segment could fit in relation to each person's trees is the best we can do at this point for single DNA segments.

Answer 2

You won't have a 20cM segment with the MRCA (most recent common ancestor) back 14 generations.  My experience working with similar sized segments is that the common ancestor is back about five generations.

Do any or many of the matches have trees taking them back to Ireland or Scotland in that five generation timeframe?  If so, that could be your direction, as well as a reason why you might not be finding a common ancestor.  I have a LOT of matches with large segments and we can't find the common ancestor.  What we do find is that we both/all have ancestors in the right timeframe that were in Ireland, but none of us can take our ancestors back beyond our immigrant ancestors (or their parents). . .

Answer 3

M Cole,

The clue may be in your comment "this seems to be a fairly common matching point".
Some bits of DNA are "sticky" and last a long long time, being passed on to a lot of descendants intact.

I have several segments larger than 20cMs where the common ancestor with the match has not been found within 5 generations.

One of 36cMs has a likely family in the right area that fits the triangulated known family, but any connection has to be back at least 8 generations, and has not yet been found.

Looking at GEDMatch I find that those matches that GEDmatch shows with their total DNA shared = to the largest segment (this at defaults on the One to Many and One to One report) are often longer lasting segments passed down intact.
Particularly when that is coupled with a LOT of matches at the same spot.
I generally move on to the more solvable triangulated groups and only plug away at looking for other connections on such segments when someone new joins the group.

Lorna
http://LornaHen.com

Comments

My experiences are similar as noted elsewhere

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Roberta Estes has a nice blog article that discusses "sticky" segments. Just Google "sticky segments DNA" without the quote marks, and you will find her blog DNA eXplained as well as several other informative articles.

Pam Tabor

Answer 4

https://isogg.org/wiki/Autosomal_DNA_statistics.  Scroll down to see the chart of most common relationship by cm.