Saturday, September 7, 2013

DNA Separating the Wheat from the Chaff

Alway an exception to the DNA rules an 11th cousin shares a large block?

My ancestors being mostly farmers would appreciate the wheat from chaff analogy. Working with my Family Finder results is quite complicated when it comes to the analysis of the results and what all the numbers mean. The long list of matches most of us get leads to many hours pondering about our possible relationship? A couple were very apparent due to the fact the person had posted family names and a gedcom. A gedcom is necessary when it comes to determining where our common ancestor is. Some of my best matches haven't shared any names or a gedcom? That's pretty is frustrating. I find that I can sometimes find their family tree posted elsewhere by using their email address in a Google search.
This week was a great week from a genealogy DNA standpoint (heat wise it waUrmey. We were estimated to be 3rd cousins by Family Tree DNA, but we are actually 4th. We checked the chromosome browser and we both share a segment of DNA in the same place as another match we both share. Using the "in common with" feature, and the chromosome browser helped me figure out a couple connections this week. I also found another Poteet connection this way.
Longest Block shared with Sophia
Probably French Canadian
s bad in So Cal). On August 26th and 29th Family Tree DNA did some large uploads of new matches. I had a dozen or so new matches. I sent out emails to my closest new matches, and I heard from one. We were able to find our common ancestors who were Jesse Callahan and Eve
My 3rd cousin Sophia uploaded her 23andme test results to gedmatch so we could compare. We were both blown away by the amount of DNA we shared! We shared a total of 149 cMs, and a longest block of 69 cMs with 17,000 snps . I did some more reading about analysis of autosomal results. What I've come away with after spending many hours in reading and analysis the last few weeks is:

  1. The most important thing to look at is the length of the longest shared block. The longer the segment the closer the common ancestor. We really see this demonstrated with the 69cM block Sophia and I shared. The next longest block I share with someone is 25 cMs. Family Tree DNA ranks matches according to the size of the longest block as the default (my 4th cousin, Callahan match, shared a longest block of 17.06 cMs with 3248 snps, and 44 total cMs. My 5th Cousin Nan shares a longest block of  12cM with 2800 snps, and a total of  39 cMs).
  2. If your most recent ancestor was shared in the genealogical time frame you will generally share 10 cMs and 1000 snps on a block of shared DNA.
  3. Use "in common with" and the chromosome browser to link possible shared ancestors
  4. You can share smaller blocks and they can be IBD, but you need a long paper trail to prove your relationship
So exactly one year later I've worked out the common ancestor for about 12 matches. I currently have 190 odd matches with no clue to our shared ancestor? I'm making a spread sheet to sort things out better. I look at gedcoms for my matches no matter how distant our possible relationshp is estimated to be (as Family Tree DNA says these matches can be as many as 20 generations back). It's easier to make connections with the highest ranked matches, so I focus on them. Although, I should probably think about distant matches since my brickwall lines are back in the 18th Century.

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