I wanted to add two thoughts: 1. Generally the larger the shared segment is, the closer the relationship (not pushing back farther) 2. An X-match with no shared atDNA, is roughly equivalent to a Chr 5 match with no other shared atDNA - it happens all the time. I also note that the different tools at GEDmatch have different algorithms. The best advice I've heard is to rely on the one-to-one data. Jim Bartlett [email protected] -----Original Message----- From: Ann Turner <[email protected]> To: Wesley Johnston <[email protected]>; DNA Genealogy Mailing List <[email protected]> Sent: Sun, Oct 8, 2017 10:29 pm Subject: Re: [DNA] Working with high largest X-matches on GEDmatch I'm wondering if there's some sort of database error here. I'd suggest sending your findings to [email protected] Ann Turner On Sun, Oct 8, 2017 at 5:54 AM, Wesley Johnston <[email protected]> wrote: > I am looking to see just what a high X-match that has no shared atDNA can > tell us about shared ancestral lines. Clearly this is pushing back very far > in time. I am using a female kit that is of pure Czech ancestry as far back > as we know (roughly in the late 1600's and early 1700's). On her 6-gen > X-inheritance chart, we know at least the name of 9 of the 13 X-ancestors > and know the spouse's surname in all 13. So the challenge of finding a > matching line is narrowed to Czech or Slavic lines. > > But that is not the problem that I am running into on GEDmatch. The > problem is that different GEDmatch tools are presenting conflicting > results. The kits come from different test companies, so that I am not sure > if this is a factor. > > To allow anyone wanting to repeat this to do so, I am going to include kit > numbers. The kit of the focus person is T685302. > > Here are the steps that I have done. > 1 - Identify X-DNA matches, sorted by largest X-DNA cM. > I used the free one-to-many and then sort descending on largest X-DNA cM. > Setting aside immediate family members, the top 10 matches range from 16 to > 21 largest X-DNA cMs, and all but the top one show zero total atDNA cMs > (the top one M052745 has 5.1). > 2 - Do a one-to-one X-DNA comparison of some of the top 10. > Clicking on the X in the X-DNA Details column, you would expect to see the > same information shown in the one-to-one comparison as in the sorted > one-to-many. And in some cases, the X 1:1 does show about the same (an 18.8 > largest X-DNA A916434 on the 1:many shows as 18.7 on the X 1:1). But in > some cases, the X-DNA 1:1 shows zero shared X-DNA (an 18.3 largest X-DNA > M131017 on the 1:many shows as no match at all on the X 1:1). > So I tried dropping the SNP threshold with M131017 on the X 1:1 to 100 > SNPs and the minimum cMs to 1 (which should not be necessary if there is > indeed a largest X shared 18.3 cM as the 1:many list shows). And this does > generate a list of shared ranges on the X, the largest of which is 5.4 cM > (162 SNPs) by cMs or by SNPs 3.5 cM (295 SNPs). So clearly this is a > radical difference from what the 1:many list is showing when it reports a > largest shared X of 18.3 cM). > 3 - Create a Tag Group for the highest matches. > I used the Tier1 1 1:many tool to create the group, setting the option to > filter by X (instead of autosomal) with offset 0, limit 500 (the minimum > allowed) and cM size 3 (the minimum allowed. This gives a completely > different (from step 1) top 10 list. Where the free 1:many had largest cMs > of 16 or more for the top 10, the Tier1 1:many showed no one (other than > close family) with a largest cM of more than 11.2. > > So I then tried the Tier1 1:1 on autosomal and setting to the minima and > sorting on the largest X (trying to re-create the list in step 1. But since > the Tier1 1:1 does not include atDNA matches of zero, neither one of these > methods was able to find the same high largest X but low atDNA matches that > the free 1:1 tool found. > Nevertheless, I created a tag group of those X-matches (from the atDNA > version of the Tier1 1:1) who had the highest total X cMS (20.0 and above, > minus immediate family). This tag group has 20 members. > 4 - Run the X-DNA Matrix Comparison for this tag group. > This was a real shock. Since the tag group had been specifically created > from a 1:many list of the X-DNA matches of T685302 who had 20.0 or more > total cMs, you would expect to see a number in every cell of T685302's > row/column. But 8 of the 19 cells are empty. Going back to the Tier1 1:1 > list used to create this tag group, one of these empty cells is the second > highest total cMs (40.8), but the largest for this match is just 6.4 and > thus under the threshold of 7 cM for the matrix, which explains the empty > cell. > > > The bottom line seems to me to be implicit and explicit threshold > differences among the tools. I really would like to work with the top 10 > from the free 1:many, but even the free X 1:1 conflicts with some of the > ones shown in that list. And the Tier1 1:many does not even discover these > matches (apparently because the Tier1, even when choosing the X option) > still has some implicit atDNA threshold). > Choosing these low-autosomal / high largest X matches as a target is a > challenge in the first place. But the GEDmatch tools make it extremely more > challenging when they do not allow exploration of those matches. > Clearly having more ability to twist the knobs on GEDmatch would help. But > the fact that (step 1 and step 2) there are very significant conflicts > between one GEDmatch tool showing 18.3 cM of a largest shared X and > another showing nothing more than 5.4 cM for the same two people seems to > be more than just finding the right settings of the knobs.