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autosomal DNA testing - Fazed over Phasing

Replies: 11

autosomal DNA testing - Fazed over Phasing

Posted: 1377061479000
Classification: Query
Edited: 1377088174000
Last night I encountered the following situation at FTDNA:

My Mom matches a mother and a daughter. Let's call them Winnie (the mother) and Paulette (the daughter).

Mom, Winnie, and Paulette all match on the same segment of chromosome 21.

I compared Winnie to my grandmother (Nonna). They don't match, so I assumed (ha!) that this match is on my grandfather's side.

However, then I noticed Paulette among Nonna's matches. I assumed (ha HA!) that it was a connection on another line. So, I was very surprised to see that Paulette and Nonna match on that same segment of chromosome 21.

So! Mom, Winnie, and Paulette match on a segment. Mom, Nonna, and Paulette match on the same segment. Nonna does not match Winnie.

In all cases, the shared segment is 10 cM, just over 5 Mb, and over 1000 SNPs.

I'm guessing that FTDNA uses a 5 Mb cutoff like Ancestry does, and maybe through general flukiness, Winnie just missed that cutoff when her test was compared to Nonna's? (I'd ask this on the FTDNA forums, but I like it better here.)

Lesson learned: I'm going to be all the more cautious now about taking sides if the match is low in Mb or if I'm basing my assumptions on the absence of a match. If Winnie's daughter hadn't tested, I'd be running down the wrong trail entirely.

(Unless this is all IBS chaff and I shouldn't be running down any trails at all.)

P.S. Amusingly, I only match Winnie at FTDNA. I did consider this scenario:

Mom, Winnie, and I match on chr 21 on my grandfather's side.
Mom and Paulette match my grandmother at, eerily, the exact same stretch of chr 21 (presumably through Paulette's father).

However, I do match Paulette at GEDmatch (Winnie's not there), so I assume it's all just a matter of unlucky wobbles in the algorithm and not an incredibly odd coincidence. :)
SubjectAuthorDate Posted
Shari S. 1377061479000 
jbarry6899 1377085172000 
Shari S. 1377097261000 
sherburl 1377098473000 
jbarry6899 1377099881000 
Shari S. 1377105995000 
jbarry6899 1377112678000 
sherburl 1377118320000 
jbarry6899 1377120229000 
Ann Turner 1377128046000 
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