Shared segments, longest segment

[rbehra]

My most recent match appears in the list of matches showing 15.4 cM of shared DNA. When I click on it, it shows:

Shared segments: 1
Longest segment: 7.7 cM

Yet on the right, in the large circle, it says:

0.21 %
Shared DNA
15.4 cM

Why the discrepancy?

Robert Behra

[jyb]

The longest segment (7.7 cM) is included in the total shared DNA (14.4 cM).

[rbehra]

Thanks for the reply, Jean-Yves, but it still doesn't make sense. If there is, as you'll see on the attached screenshot, only 1 shared segment of 7.7 cM, why does the other display say the amount of shared DNA is twice that, 15.4 cM?

Robert Behra

[jyb]

This segment is fully identical. Its length is therefore counted twice (once for the segment of the paternal chromosome and once for the maternal chromosome) to get the percentage of shared DNA. Usually, full identical segments are only found between siblings. Given the size of this segment, it's a false positive and it was by chance that the genotypes of the two DNA files were completely identical at these places. It is rare but possible.

[rbehra]

Thank you, Jean-Yves. That is a fascinating explanation. The "false positive" is easy to believe in this case, since the supposed match appears to have 100% Scandinavian ancestry, and I am unaware of any recent ancestors from that part of Europe.

Robert Behra