Wednesday, August 22, 2012

Autosomal DNA What does cM stand for? Plus my current match total

 If you share a block in  the same location with  several  matches you  likely share a common ancestor. This is true even if the blocks overlap each other.
The easiest way to understand the FT DNA autosomal results is to start with a definition of cM. It's a genetic measurement and it measures genetic distance. According to Wikipedia "centimorgan (cM) is a unit that describes a recombination frequency of 1%. In this way we can measure the genetic distance between two loci, based upon their recombination frequency."  When you are using the Chromosome browser at FT DNA the higher the cM number you select the larger the segments will be for comparison. 5 cM and over signifies a relationship,  and over 10 cM conclusive proof of a relationship. The shared cM numbers on your match pages present a different indicator. The higher the number of shared cM in the match section of your results signifies a higher likelihood of recombination for the shared block. The higher the likelihood of recombination the more likely your match is a close relative. Lower shared cM numbers mean that segment does not recombine as often so your match may be more distant in time. I found this page at FT DNA to be the best explanation of the whole autosomal DNA process http://www.familytreedna.com/faq/answers/default.aspx?faqid=17

So the most important numbers to remember are:

  1. The longest block. The longer the block the more likely you are to be related.
  2. Shared cM. The higher this number the more likely your relationship is recent (closer cousin)  a lower number could mean your match shares a more distant relationship with you (more distant cousin).
  3. The Chromo Browser setting number relates to the size of the shared block. The higher the number you set it to the larger segment will be for comparison to one or more matches. You may share many smaller segments of DNA with someone also,  so you can set the browser to show segments as low as 1 cM. 

Chromo Browser to compare shared blocks.

So far I've heard from 21 of my 113 matches. If my matches did not have an extensive gedcom file posted I sent them an email with all of my known ancestral names. The return rate of my emails is a little over half of those I sent out. I've only be able to establish a definite cousin relationship with one of the respondents.

So far out of 113 matches I have been able to confirm that seven are cousins. Five are related on my French Canadian side and through my father's side. Only two are related to my mother's side.  I see a few other possible connections, but for the vast majority can't find the common ancestor yet. I've confirmed  many French Canadian cousins because so many early records survived in Quebec. I am having difficulty connecting with US cousins because my ancestors lived in the South where so many records were lost. 

2 comments:

Nan said...

Am I your only match on your mother's side?

Annette said...

Oops!! Better correct that. I have two. That sounds better. Thanks Nan!