Archiver > AUTOSOMAL-DNA > 2014-04 > 1396551256

From: Don Worth <>
Subject: [AUTOSOMAL-DNA] The significance of segment overlaps
Date: Thu, 3 Apr 2014 11:54:16 -0700

I think this question has been asked in various ways in other forums, but I'm trying to get a more precise understanding before I start promulgating incorrect information to groups that I'm presenting to over the next few weeks.

We all know about John Walden's little table that indicates the likelihood that a segment of a given length is IBD or non-IBD (IBS). For example: a 7 cM segment is 30% likely to be IBD while a 5 cM segment is only 5% likely to be IBD.

Can anyone opine on these questions on the basis of their experience?

1 - Does the fact that two segments overlap and the corresponding matches are ICW affect the probability that a segment is IBD? For example, if you have a Triangulated Group (TG) with two or more overlapping segments in it that all ICW, and you find a, say, an overlapping 7 cM segment that ICWs as well - is it more likely than 30% to be IBD? What about a 3 cM segment that is completely overlapped and ICW with a TG?

So how small can a segment be and still participate in a TG?

2 - How much must a segment overlap another to be considered "overlapping"? (assume the matches involved ICW) Is the area of overlap governed by John Walden's predictions? Or do you consider the entire length of each of the segments that partially overlap one another?

So how much of an overlap area must there be for a segment to be considered overlapping another to form a TG?

I had been working on the assumptions that:

A. Short segments are more likely to be IBD if they are part of a TG (although I don't know that I would go as far as to say a 3 cM segment was IBD - especially since the reason you ICW the owner of that segment is probably due to a longer segment somewhere else that could be associated with a different ancestor).

B. The overlap must be sufficient to stand on its own as a likely IBD segment (although, since it is part of a TG, it might be more likely IBD). The size of the portions of segments that don't overlap should not be considered when declaring a TG. (Although, presumably they could represent differing portions of a longer segment (that you have) from the common ancestor.)

I would appreciate some feedback on this before I make a fool of myself in my slide show!



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