Too Many DNA Matches:
A Catalogue of Discovered DNA Regions

by Wesley Johnston
begun 8 Dec 2018, inspired by Diahan Southard's presentation at the 2018 Institute for Genetic Genealogy (i4gg) conference in San Diego
last updated 20 Dec 2023 - add new region on chromosome 2
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Copyright © 2023 by Wesley Johnston All rights reserved

I am definitely NOT the best person to do this catalogue. But at this point, no one else is doing it in a way that allows for ready identification of a specific area that might come up in a research project.

The ISOGG Wiki "Identical by descent" page's section on "Excess IBD sharing" comes closest, and DNAPainter uses that page's problem regions to warn about potential problems when you are doing chromosome painting in DNAPainter.)

Family History Fanatics also have a nice concise page on pileups which builds on Debbie Kennett's 22 Jan 2018 post "Small Segments and Pileups.

I would really like to include a web page about GEDmatch Q-Matching which addresses the SNP-poor regions, but I have yet to find a good web page on Q-Matching (although the Family History Fanatics page noted above does address it well though not completely). There is a good 2 Aug 2020 statement by Wilbon Davis in a reply a query on the GEDmatch forums that is worth quoting "Be sure to recognize that Q only helps eliminate segments that are accidental matches. Rely more heavily on cM lengths for estimating kinship and IBD filtering."

So I am doing what I can, with what knowledge I have.
-- Wesley Johnston


Overview

When examining DNA matches, some regions of shared DNA come from a shared common ancestor. These are called IBD (Inheritance by Descent). But others come from various other situations that are not indicative of the DNA coming from a recent common ancestor. These are in a group of match situations called IBS (Inheritance by State).

While there are more forms to consider, I will leave it at this being a catalogue of excess IBD of cases of too many DNA matches on the same region of the same chromosome. I had originally thought to include IBS due to a shared ancestral population (IBP), but those are not included.

I do think that it is very important for qualified researchers to research each of these and see just what they tell us. They are not useful for identifying common ancestors, and the catalogue is helpful for identifying when you have stumbled onto one of those cases, which is the main purpose of the catalogue. But I suspect that they have importance that we need to understand. This is fertile research ground for some enterprising well-qualified researcher seeking to "boldly go where no [researcher] has gone before".

At this point, I do not plan to classify any of these, as to which type they are or as to what they might mean. I simply want to capture all cases where this is found. I began (7 Feb 2021) a C22 Pileup Project, but I have not worked on it for a very long while now, and it really never got off the ground.


KNOWN REGIONS

Chr
StartEndLen (cM)Source - Details
1
798,3593,035,2277.3 cMThis is a region that may be Scots-Irish DNA that shows up in some of the Johnstons of North Ireland kits. This may be the same as the region identified below for the Lake family which is also Irish or Scots-Irish DNA with which it significantly overlaps.
1
~838,000~5,600,000~7-10?This is a region that may be Irish DNA that shows up in some of the Loyalist Lake Family kits. It was identified by Craig Kanalley. So far, the kits that triangulate on this all have Irish or Scots-Irish ancestry, and some of them definitely did not inherit the region from their Lake ancestors.
1
118,434,520153,401,1089.95https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
2
~16,300,000~28,800,000~10.5This is a region that appears to be a possible Slavic excess IBD region.
2
85,304,24399,558,0136.53https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
2
132,695,025141,442,6369.16https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
2
192,352,906198,110,2295.04https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
6
29,750,00033,100,000 https://dx.doi.org/10.1371/journal.pgen.1004144 - HLA Region
6
~70,300,000~99,200,000 21.9 cM/ 13056 SNPs https://familylocket.com/the-irish-endogam-ish - County Cork, Ireland, pileup identified by Heidi Mathis
8
9,600,00010,400,000.https://www.genetics.org/content/186/1/295 - Charts 6 and 7: " On the basis of the simulations we used a conservative critical value requiring extreme regions to have more IBD than the most extreme region observed in any of the simulations (1.34% IBD sharing). Only two regions fulfilled this very conservative criterion: the HLA region on chromosome 6 and a region from 9.36 to 10.4 Mb on chromosome 8 (see Figure 6 for a closeup of the IBD sharing in these regions). The amount of IBD sharing in the two regions is not likely to be the result of neutral evolution, at least not under the demographic models used here. "
8
10,428,64713,469,6937.96https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
9
38,293,48372,605,2618.15https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
10
44,555,09353,420,1887.58https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
15
20,060,67325,145,26010.46https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
15
27,115,82330,295,7509.29https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
16
19,393,60624,031,5566.18https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
17
59,519,08364,970,5316.23https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
17
77,186,66678,417,4785.66https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
21
16,344,18678,417,4786.91https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)
22
16,051,88125,095,45120.82https://dx.doi.org/10.1371/journal.pgen.1004144 - Table 3 (Build 37)

Chromosome 11

"High IBD sharing signals were also seen on each side of the centromere on chromosome 11. These regions contain clusters of olfactory receptors."
[source:
ISOGG Wiki on Excess IBD, citing "Albrechtson A, Moltke I, Nielsen R. Natural selection and the distribution of identity-by-descent in the human genome. Genetics 2010; 186 (1 ):295-308. See Table 2 for the genomic positions of the regions on the different chromosomes where the peaks of IBD sharing were found."]


Contact Information

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Copyright © 2023 by Wesley Johnston
All rights reserved