Here is my exchange with Ann Cousin aka DNACousins.
I asked for clarification on some things to make it clearer but this morning I told her it was not needed. I understood why she answered as she did but just reading the response.
1. Conceptually, a match for a son not found in his mother can be attributed to his father. This includes IBD and IBS, but not IBD. I have no problem limiting those matches (without triangulation), to only those that include an IBD segment, which is all I initially intended.
2. Given the answer to #1 only requires a match, it indicates that triangulation is not necessary. Since triangulation is only used to find common ancestors for those without a tree, she interpreted the question that way.
Do I really have to ask Ann to clarify her last statement by telling her that the Wikitree technical group believes that triangulation is used for something other than finding the common ancestor? Do I really have to say to her that the Wikitree Technical members are not convinced her answer to #1 because "If you want to know which side of your tree somebody is on, you need a triangulated 3-way match with another cousin. "
I tried to ask the question so not to bias her answer but I should have noted that Wikitree places a triangulation requirement on more than finding common ancestors.
From: Ann Turner
Sent: Saturday, June 04, 2016 4:32 PM
To: Kenneth Sargent
Subject: Re: I am hoping you will clear up a disagreement.
I've been wishing I could spend more time on WikiTree, but it seems like there's always something else demanding my attention.
1) Conceptually, a match for a son not found in his mother can be attributed to his father. There are a couple of "gotchas", though. The segment must be long enough that you can rule out a coincidental match. There's no consensus on how long that should be. And there is also a possibility of a false negative in the mother, e.g. at FTDNA (which requires a total of 20 cM, including small 1-3 cM pseudo-segments), AncestryDNA (with its TIMBER algorithm discounting some segments) and 23andMe (with a cap on the number of DNA Relatives). GEDmatch lets you look at everyone through the same lens.
2) There's also no consensus on whether you "need" a triangulated group. AncestryDNA uses more of a network approach. I wrote up some material about how difficult it is to assemble TGs here: http://tinyurl.com/TheTroubleWithTriangulation. But if you have the good fortune to get a triangulated group with pretty robust segment sizes, I do think it's possible to attribute it to a specific ancestral couple if it's not too many generations back. When you go back many generations, that brings up the possibility of multiple lines of descent.
Hope that helps,
On Sat, Jun 4, 2016 at 9:48 AM, Kenneth Sargent <firstname.lastname@example.org> wrote:
I’ve been spending too much time on Wikitree, devoted almost entirely to the discussions on DNA. I suspect that Wikitree is the best source for publicly available documented trees but the discussions are not at the level as 23andme used to be. I was hoping to ask you two basic questions and get your permission to post your response. We are discussing the “mathematics of genetic genealogy”.
Your responses to these questions could significantly affect how Wikitree users think about how to use DNA in their research.
Scenario: We have the raw data for a mother and a son available to us for customization. There are matches to the son, that are not matches to his mother. More specifically for these matches, the segments are shared with the son, but none are shared with the mother. Since the data can be phased, I am presuming the process could phase the data first.
1. Is it possible, using the data available, in these cases, that a match to the son, and not the mother, is probably related to the son via the father?
2. Do you agree “If you want to know which side of your tree somebody is on, you need a triangulated 3-way match with another cousin. “ FYI – a triangulated 3-way match” means part of a Triangulated Group. You don’t have to go further than yes or no, but feel free to comment.