Autosomal Triangulation and Family Finder [closed]

+12 votes
420 views

The WikiTree Help on DNA Confirmation states, under Autosomal Triangulation, "Family Finder does not allow you to see exactly how your matches match each other on specific segments. "In Common With" is insufficient.". This seems to ignore the Chromosome Browser function. Was this feature added after the Help was written, or are there some issues with it that don't make it reliable for triangulating? 

closed with the note: We have the answer
in The Tree House by Living Hampson G2G6 Pilot (112k points)
closed by Living Hampson
Good question, Chris.  I think Peter Roberts will probably need to respond to this as I think he wrote that instruction.
ICW (In Common With) at FTDNA frustrates me. The only chromosome browser that displays true triangulation is at 23andme. And of course, GEDmatch.
I'm putting this in a comment because it relates to a number of the answers already given - thanks to all. The piece that only just sunk in for me was the fact that we have two copies of each chromosome, so there is always the possibility that person A will match on one copy and person B with the other copy, but both in the same segment.

4 Answers

+10 votes
 
Best answer
It is not so much that ICW is insufficient rather that ICW from a single perspective (let us say account A) is insufficient as it only provides confirmation that A matches B and C on the same segment for a sufficient length but says nothing of whether B and C match in the same place.

 Although it would be best to have DNA results from A, B, and C on GEDmatch, if that is not the case, one can rely on ICW if one were to contact either one of B or C to get them to use their account to confirm that B and C match on the same segment with the range provided by A and for a sufficient length.

The alternative is to get BOTH B and C to download their DNA results to a file and load it on GEDmatch.  This, ultimately is preferred but does require one to contact BOTH matches (unless one of them already is on GEDmatch) and requires more effort than merely checking for the ICW between B and C.
by Living Anderson G2G6 Mach 7 (78.6k points)
selected by Living Anderson
To me, the FTDNA ICW functions much like Ancestry DNA' s SHARED MATCHES tool....they match but one does not know if they match on same segments.
Ancestry does not have a chromosome browser so one can neither determine which chromosome has the match nor the start and stop locations.  ICW itself will not reveal that but the FTDNA chromosome browser will.  As I mentioned, that is not enough for triangulation as it only says that A matches B and A matches C.  HOWEVER, one can contact either B or C and ask them to use the chromosome browser to check on the match between B and C.  If the match coincides with the matches from A to B and from A to C (for a minimum of 7 cM), then the triangulation is successful.
+10 votes
Chris,

All the chromosome browser allows you to see are how YOU match the matches.

The statement is accurate, it does not allow you to see how your MATCH matches the other matches.

95% of the time (I think this was Jim Bartlett's estimate - segmentology.org), yes, if you can see that you and the other matches are at the same spot of the same chromosome AND ICW each other, you are reasonably safe to assume triangulation, but for 100% you really do need to check that the matches do actually match each other there.
by Lorna Henderson G2G6 Mach 3 (30.7k points)
+6 votes

As Lorna has written, you can't see how two people that match you on the same segment and chromosome, match each other.

Let's name them Laurel and Hardy. You match both of them on chromosome 1 from 30-44 mBp.

With no tool at FTDNA you can see where exactly Laurel and Hardy are matching each other, you can only see if they match!

It's also explained with other questions on my extended FAQ page here: https://www.dnagenealogy.tools/extended-faq

by Andreas West G2G6 Mach 7 (74.8k points)
+6 votes
Chris, take those same ICW matches and take them further using the MATRIX tool at FTDNA.
by Maggie N. G2G Astronaut (1.3m points)
Maggie,

The Matrix tool is also only In Common With
True, Lorna, but one can eliminate some ICW matches from selection process.

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