Shared cM Question

[rebeccaclaire86]

This is probably a stupid question, but I'll ask anyway!  I took the Family Finder test through FTDNA and my results have just come back this morning, I've been enjoying playing around with them and have already spotted a few possible links on there which is exciting.  However I've sorted by shared cM and the top match who I share 55cM with is coming up as a remote cousin, however my known second cousin who is the third match who I share 53cM with is coming up as a 'third to fifth cousin'.  How is it you can share more cM with someone but have less chance of being related?

(Probably a stupid question, and I should probably not admit that I work as a Biology teacher - in my defence my background is in Marine Biology and Ecology!!)

[rebeccaclaire86]

I may have answered my own question here; is it to do with the lengths of the sections of DNA that are in common as well as the total number?

[DevonCruwys]

The relationship predictions are worked out on the basis of the amount of DNA shared and the number and length of the DNA segments shared. FTDNA have some odd matching criteria and include small pseudosegments under 5 cMs in the total cM count so that might account for some of the difference. The following pages in the ISOGG Wiki might help:

http://www.isogg.org/wiki/IBD

http://www.isogg.org/wiki/Autosomal_DNA_statistics