Show Posts
1
Ancestral Family Tree DNA Testing / Scope and limitations of DNA matches
« on: Sunday 22 June 25 09:23 BST (UK) »
I thought I understood Ancestry's DNA matches. Turns out that was something missing in my understanding. (important bit in red)
I have spent around 6 hours carefully researching ALL the people for whom I have DNA matches AND either trees I've researched myself or "other peoples tree" on ancestry. This means I (or ancestry) know the common ancestors for the matches. I have then labelled (in my own database) the set of g-g-grandparents (a pool of 16) from which each matching person descends.
The point of this exercise, beyond recording information, was to allow the following process.
If I take a person descended from (say) ancestors 5,6,7,8 and get a list of their shared matches...
and then get a similar list for a person descended from only 7,8...
Anyone in the 5,6,7,8 list who is NOT in the 7,8 list must be descended from 5,6.
Right? So I can manipulate shared match lists to gain more detailed knowledge. Cool.
But it turns out DNA matching doesn't work like that.
Since you only match on shared fragments, who you match with not only depends on the ancestors they're descended from, but which particular fragments you (and they) have from those ancestors.
The upshot was, on my first trial run, the 7,8 list included a person who I KNOW to be a 7,8.
But she was NOT in the 5,6,7,8 list.
So my deductive process, for which I had high hopes, doesn't work.
I have spent around 6 hours carefully researching ALL the people for whom I have DNA matches AND either trees I've researched myself or "other peoples tree" on ancestry. This means I (or ancestry) know the common ancestors for the matches. I have then labelled (in my own database) the set of g-g-grandparents (a pool of 16) from which each matching person descends.
The point of this exercise, beyond recording information, was to allow the following process.
If I take a person descended from (say) ancestors 5,6,7,8 and get a list of their shared matches...
and then get a similar list for a person descended from only 7,8...
Anyone in the 5,6,7,8 list who is NOT in the 7,8 list must be descended from 5,6.
Right? So I can manipulate shared match lists to gain more detailed knowledge. Cool.
But it turns out DNA matching doesn't work like that.
Since you only match on shared fragments, who you match with not only depends on the ancestors they're descended from, but which particular fragments you (and they) have from those ancestors.
The upshot was, on my first trial run, the 7,8 list included a person who I KNOW to be a 7,8.
But she was NOT in the 5,6,7,8 list.
So my deductive process, for which I had high hopes, doesn't work.
Think I have sorted it now .Thanks