Timeline for Interpreting matches with low cM values and high SNPs (or vice versa)?
Current License: CC BY-SA 4.0
9 events
| when toggle format | what | by | license | comment | |
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| Dec 6, 2018 at 15:28 | comment | added | Cyn | Some of us do not buy the IBC vs IBD methodology (the naming of it and the assertions about segment size) but that's a longer discussion for another post. Thanks for coming back with the update. | |
| Dec 6, 2018 at 7:41 | comment | added | cleaverkin | OK, I found the reference and I did mis-represent it. It was from Diahan Southard's "Genetic Genealogy: Advanced" webinar at Legacy Family Tree Webinars. The figures she claims are that at 7 cM, segments are about 80% likely to be IBC, at 15 cM about 80% likely to be IBD, and at 10 cM about 50% either way. She was referring to all segments, not just largest segments. | |
| Dec 4, 2018 at 20:07 | comment | added | cleaverkin | Maybe I misremembered, it might be largest segment. It was one of the Legacy Family Tree webinars, I'll see if I can dig up a reference. In my case, I was referring to matches with 20-25 total cM, so largest would also fit that scenario. | |
| Dec 4, 2018 at 20:03 | comment | added | Cyn | Are you sure that's not the threshold for the largest segment? Most of us in endogamous communities say 20 for the largest, but 15 is not unreasonable, especially for non-endogamous. The 5 or 7 cM segment minimum I'm talking about is for any segment used in adding up the total cM match. For a close match, there would need to be one or more segments significantly larger than that. | |
| Dec 4, 2018 at 20:03 | comment | added | Cyn | I'd be interested to see more on that. She is going against all the genetic genealogy wisdom and leaders that I've seen. 7 is pretty standard with 5 common. I may have seen something higher than 7, but not higher than 10. 15 is rather shocking, actually. | |
| Dec 4, 2018 at 19:57 | comment | added | cleaverkin | In one of Dianne Southard's webinar's, she suggested that 15 cM is actually a better threshold - that segments below 15 cM are about 80% likely to be IBC and above 15 cM about 80% likely to be IBD (I'm sure it's actually more subtle that that). This also seems to be in line with my personal experience. | |
| Nov 29, 2018 at 4:51 | comment | added | lkessler | However, at GEDmatch it is important to note small SNP counts. Different companies test different SNPs, and if there is a low SNP count for a match, say under 500, then it is a low quality match with a good chance of being a false positive no matter how many cM the match might be. | |
| Nov 28, 2018 at 22:39 | comment | added | Jan Murphy♦ | Your answer could be improved by adding links to posts explaining why small segments are usually ignored, such as Blaine Bettinger's A Small Segment Round-Up: thegeneticgenealogist.com/2017/12/29/a-small-segment-round-up | |
| Nov 28, 2018 at 20:15 | history | answered | Cyn | CC BY-SA 4.0 |