Saturday, November 07, 2015

Comparing Shared Centimorgans

Thursday Angie Bush posted on Facebook's ISOGG group that Ancestry DNA was now reporting the total shared centimorgans. Everyone was excited about this development, but people began to notice discrepancies which were being attributed to algorithms.

I decided to take a match for a known second cousin once removed and compare the totals reported for Ancestry and GEDmatch, using default settings. For privacy reasons, I'm not including the match's name or GEDmatch kit number.

Ancestry reported 96 cM across 6 segments. GEDmatch returned 6 segments. If you look at the bottom, it shows that the total of segments > 7 cm was 98.3 cM. This is pretty close. While I'd love to know which 2.3 cM Ancestry DNA did not find valid, it is not something over which I will lose sleep. Yes, I'd love for Ancestry to give me the segment data. I tell them I want a chromosome browser every chance I get.

1 comment:

Terry Stewart said...

I did the same comparison ...and got at least a 20pt discrepancy... big difference no idea what's going on here. Any opinions?