When I say "legitimately" match, I mean an IBD segment that would meet the company's matching threshold, not a segment made up of stretches of IBD and IBS pieced together. I'm not saying that it is impossible for a match over ~7 cMs to be IBS, but very unlikely. (I was going to write ~10 cMs in my response, but thought that might be a bit conservative.) I know that it is theoretically possible, but in my thousands of hours of atDNA research, I have yet to see proof of one. Can you please show me an example of an IBS match over 7 cMs? (I know that Ann Turner has seen lots of them between 5cMs-7cMs. We'll see if she responds with a larger one.) If enough of a portion of the segment is IBS that it doesn't show up in your parent (i.e.- make the company's threshold), then it will obviously reduce the authentic match to a point that is not worth pursuing. I have chased far too many matches under 7 cMs to recommend doing the same. How does one's parents being distantly related "make it more difficult to draw conclusions from lack of match"? Perhaps, if they are related, it might make it more difficult to draw conclusions from the PRESENCE of a match as to which side you inherited the DNA, but it doesn't matter for triangulation purposes with your match if the common ancestor appears in both sides of your family trees. I don't totally understand the point of your response. I am attempting to give simple advice that works in the vast majority of cases. There are always theoretical exceptions and if we went by those, we would never get anywhere with autosomal DNA matching for genealogy. Thanks, CeCe   www.yourgeneticgenealogist.com www.studiointv.com > Date: Tue, 15 Nov 2011 16:22:00 -0600 > From: [email protected] > To: [email protected] > Subject: Re: [AUTOSOMAL-DNA] Fundamental autosomal DNA question > > Not entirely true. IBS can occur for segment over 7 cM, but are more > likely with the shorter segments. > > Meanwhile, even if IBS, the majority of the segment could be IBD and as a > result appear to "skip" generations appearing for both a grandparent and > grandchild, but not the parent/child in between them. And in some cases, > it may be an even larger segment share for the grandparent. > > Of course it also possible, and more common than many may think, that a > persons parents may be distantly related which may make it even more > difficult to draw conclusions from lack of a match. > > > > > On Tue, Nov 15, 2011 at 4:02 PM, CeCe Moore <[email protected]> wrote: > > > > > You cannot legitimately match with someone who shares an ancestor with > > your mother's line without her showing the match as well. In most cases, if > > you have a match that your mother doesn't have, you can safely conclude > > that they are from your father's side. However, if the match is under ~7 > > cMs, it might be coincidental - identical by state - in other words a false > > positive, in which case it won't show up in either parent and is not worth > > pursuing. > > CeCe > > > > www.yourgeneticgenealogist.com > > www.studiointv.com > > > > > > > From: [email protected] > > > Date: Tue, 15 Nov 2011 08:44:41 -0500 > > > To: [email protected] > > > Subject: [AUTOSOMAL-DNA] Fundamental autosomal DNA question > > > > > > I’m sure this fundamental question has been answered before, but I have > > missed it. > > > > > > If both my mother and myself have tested, can I match with someone from > > a known ancestral line of hers while she does NOT? OR, would the match > > mean that I MUST be related to this person through my paternal line? > > > > > > ------------------------------- > > > To unsubscribe from the list, please send an email to > > [email protected] with the word 'unsubscribe' without > > the quotes in the subject and the body of the message > > > > > > ------------------------------- > > To unsubscribe from the list, please send an email to > > [email protected] with the word 'unsubscribe' without > > the quotes in the subject and the body of the message > > > > ------------------------------- > To unsubscribe from the list, please send an email to [email protected] with the word 'unsubscribe' without the quotes in the subject and the body of the message

One way to study the problem is with father/mother/child trio data. If the child has a match not found in either parent, it is possible (indeed I'd say likely) that the segment is Identical by State, not Identical by Descent. Ancestry Finder at 23andMe lets you look at segments down to 5 cM / 700 SNPs. For actual numbers in one case, my son has a total of 231 segments listed with names attached. 67 of those are found only in the child (29%). The breakdown by segment size is 4/76 > 7 cM, longest 11.1 cM (5%) [This seems consistent with a 95% confidence interval] 7/37 6-7 cM (19%) 56/119 < 6 cM (47%) FTDNA seems to have a cutoff of 7.7 cM / 500 SNPs (that's by empirical observation -- has anyone found a shorter longest segment?). I have data for one father/mother/child trio that seems particularly dicey, with 22/100 matches in the child not found in either parent. The segment sizes ranged from 7.7 to 12.90, with a median of 8. Other trios I've looked at seem to run about 15%. Ann Turner On Wed, Nov 16, 2011 at 1:02 PM, CeCe Moore <[email protected]> wrote: > > When I say "legitimately" match, I mean an IBD segment that would meet the > company's matching threshold, not a segment made up of stretches of IBD and > IBS pieced together. > > I'm not saying that it is impossible for a match over ~7 cMs to be IBS, > but very unlikely. (I was going to write ~10 cMs in my response, but > thought that might be a bit conservative.) I know that it is theoretically > possible, but in my thousands of hours of atDNA research, I have yet to see > proof of one. Can you please show me an example of an IBS match over 7 cMs? > (I know that Ann Turner has seen lots of them between 5cMs-7cMs. We'll see > if she responds with a larger one.) > > If enough of a portion of the segment is IBS that it doesn't show up in > your parent (i.e.- make the company's threshold), then it will obviously > reduce the authentic match to a point that is not worth pursuing. I have > chased far too many matches under 7 cMs to recommend doing the same. >