Many Matches on Same Segment on Gedmatch

[Stezinec]

Hi all,

I've noticed that when running "one-to-many" matches of Gedmatch that there are certain segments with an overabundance of matches. It seems to occur in similar places for most kits, but I wonder why this happens?

Are there certain segments of SNPs where people are just more likely to match? Still, it makes me wonder a bit about the results when I see 50 matches in a particular area with people with a wide variety of different European backgrounds. Am I just matching them on a segment that is common to many European populations? Or are these matches just identical-by-state (IBS) instead of identical-by-descent (IBD)?

Thanks for an input on this topic.

[thetick]

There are a couple of reasons I know but I bet there are more:

1) Certain chromosome regions are considered hot spots for matching since the recombination occurs less then other regions. These matches maybe much farther back then FTDNA is predicting. This happens with any DNA matching but is very common for MtDNA where "matches" can go back thousands of years.

2) Genetic bottlenecks in the population. Certain ethnic groups have these ... Jewish, Amish or any isolated group.

3) Inbreeding in the population. Again certain ethnic groups have these ... Jewish, Amish or any isolated group. Some people group 2 and 3 as the same.

4) Maybe just a member of a very common group. Examples of these are Jewish, Colonial Americans, or say British/Scottish/Irish at British DNA company.

Most likely one or more of these is contributing to the large number of matches for certain segments.

[andbro]

I have the same
Many of the blue segments are on the cusp of green at 9cm.

[Stezinec]

Thanks for the response tick. I think the one that is most relevant to my results is hot spots.

If it were due to having segments from a bottlenecked or inbred population then I think you would expect to see many of the matches being from the same ethnic group? That doesn't seem to be the case as the matches seem to be from all over the place in these segments.

I have one segment with a number of Jewish matches, so maybe that one is due to minor Jewish ancestry, but most of them seem to be quite heterogeneous.

[thetick]

Thanks for the response tick. I think the one that is most relevant to my results is hot spots.

Agreed. If you post the chromosome and range you have all these matches an expert, much more knowledgeable then me, might provide confirmation.

[Stezinec]

Here are some of the regions on Gedmatch where I have lots of matches:

Chr. 2 from 6 to 10m

Chr. 14 from 29 to 32m

Chr. 18 from 7 to 9m

Chr. 22 from 19 to 23m (a huge number of matches here)