A Triangulation CONCEPT
In the MRCA Knothole post, we looked at a funnel anchored to a genetic MRCA. In actual practice, we may not know the genetic MRCA for sure. We may well have determined several genealogy MRCAs in a TG, each from a different line (the lines don’t descended from each other). In this case you might think of multiple potential knotholes and funnels associated with each line. But ONLY ONE line can be correct. With only one or two Matches, we cannot draw firm conclusions. But as you and more Matches determine your MRCAs, a pattern usually emerges which gives you more confidence that one of the MRCAs is probably right. It may be that several Matches in a TG all agree on the same MRCA – my rule of thumb is the number of such Matches should be at least the number of Greats in the MRCA (e.g. four with a 4G grandparent MRCA (5C level). You’d want at least four cousins in the TG who are all at least 1C apart from each other (not a parent, 2 children and an uncle), who are all descendants of the same 4G grandparents*. With multiple MRCAs “walk the ancestor back” – my rule of thumb is at least a 2C or 3C (with the appropriate amount of total shared DNA) to anchor the funnel stem; and skip no more than one generation in the other Match cousins, on the walk “back”. Or use combinations of the above; or use judgment. This is your hobby, your genealogy – use your judgment! The above are my recommended guidelines to be reasonably safe – but always be prepared to adjust as new information comes in.
MY MRCA GUIDELINE SUMMARY:
- For Matches who are cousins on the same MRCA: have at least as many such separated cousins as the number of Greats in the MRCA (e.g. five 6C)
- For Matches who are cousins with various MRCAs, then walk the ancestor back: have a 2C or 3C anchor, with other cousins at different levels back to the MRCA (e.g. a 2C, 4C, and 6C)
[*NB: the four cousins from a 4G grandparent does not mean they all came from 4 different children of that grandparent. In fact that scenario is very, very unlikely. But the four cousins may be 1C or 2C or 3C to each other (not on your line). It’s possible they all came from only one child of the 4G grandparent.]
08C Segment-ology: The MRCA Knothole Guidelines! Concept by Jim Bartlett 20170910
segment-ology 
Hello jim, i uploaded my FTDNA raw data yo Gedmatch. Gedmatch says it cannot accurately predict my eye color because FTDNA holds back some of thtere SNPs Gedmatch says i have high # of No-calls i uploaded several times and always the same. Can i assume that’s due to the lack of SNPs from FTDNA. Does this. Mean the admixture isnt accurate for people who have tested with FTDNA? THANK YOU for any insight.
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Karen, It’s hard to say from your info. We all have some no-calls in our data. It appears that GEDmatch’s algorithm allows more no-calls in a Match than any other company – using the same data. I know FTDNA does not use all of the SNPs (they cull out the medical ones), but I always thought all of them were in the raw data. In any case, FTDNA’s version for genealogy, would be equivalent to 23andMe and AncestryDNA and MyHeritage, if they all used the same testing chip. The newer chips have fewer SNPs. Currently, I believe the chip used by FTDNA has the most SNPs of the big three – see the chart at ISOGG – https://isogg.org/wiki/Autosomal_DNA_testing_comparison_chart
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