Dear Jim and Shannon, There is actually a fairly strong correlation between the cMs and the number of base pairs throughout the autosomal chromosomes. Last year I downloaded a series of graphs from Rutgers University that shows this correlation chromosome by chromosome. I can't seem to find the URL where I downloaded the graphs from now, but in any case you can download them from my Dropbox account at https://dl.dropboxusercontent.com/u/21841126/map%20comparing%20chromosome%20 position%20to%20cM%20from%20Rutgers.pdf if you want to review them. The URL for the Rutgers University computation genetics web site is at http://compgen.rutgers.edu/maps. It describes some of the background there. In any case, if you review these maps closely you will see that only about 9 of the chromosomes that have portions of the graph where the slope of the curve is very flat (in other words there are a large number of base pairs per cM in these regions of the chromosome). Chromosomes which appear to have regions where there could be more than 5 million base pairs in a segment of the chromosome where there is less than 1 cM include chromosomes 1, 3, 7, 9, 11, 16, 17, 19, and 20. I suspect that there is something going on here that is more than just regions where the slopes of these graphs are relatively flat. I suspect that the phasing of the data by Ancestry.com may be faulty in some instances. In other words, some of the matches that Ancestry.com is generating may not be true HIRs that contain 5 million or more base pairs. I expressed some concerns about the quality of the phasing that Ancestry.com is doing in an analysis I did this summer which I posted at http://archiver.rootsweb.ancestry.com/th/read/AUTOSOMAL-DNA/2013-08/13765547 77. We would need to review some of the raw data files to sort this out. Shannon, if you could send me some raw data files that appear as matches in Ancestry.com but don't meet the 1 cM threshold in GEDmatch, I would be happy to review the files and determine what the lengths of the longest HIRs are. Sincerely, Tim Janzen -----Original Message----- From: autosomal-dna-bounces@rootsweb.com [mailto:autosomal-dna-bounces@rootsweb.com] On Behalf Of Shannon Christmas Sent: Tuesday, December 03, 2013 5:51 PM To: autosomal-dna@rootsweb.com Subject: Re: [AUTOSOMAL-DNA] Match on AncestryDNA, but not on GEDmatch Hi Jim, Knowing that the correlation between cM and Mbps is minimal, that was my first thought. What can anyone learn from from these kinds of matches if you cannot perform chromosome mapping? Very Respectfully, Shannon On Tue, Dec 3, 2013 at 8:30 PM, Jim Bartlett <jim4bartletts@verizon.net>wrote: > Shannon, > > It's because Ancestry's threshold for a match is 5Mbp. There are areas > where a 5Mbp segment doesn't have 1cM. > > Jim - Sent from my iPhone - FaceTime!