Sarah (Ransom) Sessions (abt. 1711 - abt. 1788)

I have only been doing this for a few years, thanks for the feedback. GEDmatch's 3D Chromosome browser seems to indicate 16.4 cM :

Segment Details: Kit1 Kit2 Chr Build 37 cM Kit Nbr. Name Kit Nbr. Name From To A541884 mack tyner A729479 *DD Amy McKay Core 23 40737186 52067217 16.4 A541884 mack tyner A729479 *DD Amy McKay Core 23 61992011 99633959 22.6 A541884 mack tyner M755857 Harriette McCollough Tuttle 23 40737186 52067217 16.4 A729479 *DD Amy McKay Core A541884 mack tyner 23 40737186 52067217 16.4 A729479 *DD Amy McKay Core A541884 mack tyner 23 61992011 99633959 22.6 A729479 *DD Amy McKay Core M755857 Harriette McCollough Tuttle 23 40389845 52067217 17.8 M755857 Harriette McCollough Tuttle A541884 mack tyner 23 40737186 52067217 16.4 M755857 Harriette McCollough Tuttle A729479 *DD Amy McKay Core 23 40389845 52067217 17.8

Ah. Now I better understand. Even though the tabbed info came through difficult to read, as you can see, I can pretty much state unequivocally that results at GEDmatch from the chromosome browser summary will almost always differ from direct, one-to-one comparisons. There are different algorithms at work, and it's been that way since the GEDmatch Genesis beta was merged into the older Classic version to become the new production website.

The Tier 1 Multi Kit Analysis tool returns the mutually shared segment as the 42362678 to 50334425 segment I noted if the Triangulation visualization option is used, but if I choose the 2D Chromosome Browser visualization instead I get the 40737186 to 52067217 loci, which is what the browser showed to you. I always default to manual, one-to-one matching. For A541884 to A729479 that shows 42362678-52067217; for A541884 to M755857 it's 40737186-50334425; and for A729479 to M755857 it's 40389845-52067217. The lowest common denominator, then--the largest low-end start and the smallest high-end stop--would be 42362678 and 50334425. To get a quick and reliable estimate of the centiMorgan value I usually hop over to the Williams Lab at Cornell: https://hapi-dna.org/query-cm/. Using 23 as the chromosome number and plugging in those start-stop locus numbers nets us 9.1cM.

I'm no authority on WikiTree DNA policy, so as far as I know that's still perfectly acceptable here for the purpose. I've argued the biology and the math in the past, but it is what it is.

Whether 9.1cM or 16.4cM, I think the basic triangulation is still valid here, and you have evidence of finding it as 16.4cM. I'll withdraw the comment and recommend you leave the value as it is. At the end of the day, centiMorgans are tricky beasts anyway. There is no measurement of them per se: they're arrived at only as an estimation of the likelihood of recombination occurring during the next meiosis, the next generation of a sperm or egg cell, within that particular range on that particular chromosome. The calculation itself has been basically unchanged since authored in 1944 by the Indian mathematician Damodar Kosambi, and takes as input data the genome reference assembly, the map of all those 3.06 billion positions, that underlies it...and for genealogy we're still using a reference assembly that's 15 versions old, released in June 2013. Bottom line is that it's...not impossible...but highly unlikely that any centiMorgan estimate we see will be precise. Too, all we use are sex-averaged cM values. The female and male genomes are very different in that regard, with the female undergoing crossover about 70% more than the male, and in turn having a a total genomic cM value that's 70% larger. And the X chromosome is even less accurate because with each father-to-daughter inheritance his X is passed down entirely intact while the mother's X undergoes recombination between her mother's and father's X contribution at about the same frequency as does Chr 7, an autosome of about the same size.

Can you tell I have the standing wordcount record on the WikiTree G2G forum for DNA questions? Sorry. Didn't mean to bend your ear so much. I just stumbled on this particular "Confirmed with DNA" triangulation because it was mentioned on the board today. So I took a closer look. I try studiously not to talk too much about WikiTree "Confirmed with DNA" policies and guidelines, and to stay away from tool "how to" stuff. But I have a bit of a background in genetics and have been involved in its use in genealogy since 2002. If you ever have a DNA question you want to toss at someone, feel free to send me a private message from my profile. All the best,

—Ed