Monday, August 26, 2019

How Many "Common Ancestors" Do You Have on AncestryDNA?

I have 325 - I had to count them all - AncestryDNA doesn't count them for some reason!  They should - it's the most useful count of all their counts.   They do provide a Filter for "Common Ancestors."

I wondered how many "Common Ancestors" I had relative to all of my AncestryDNA matches.  I have 60,721 matches (6 centi-Morgans or more) today.  

So I have 325 "Common Ancestors" provided by AncestryDNA, out of 60,721 total DNA matches.  That is 0.054% - 1 out of every 187 matches.

Next, I wanted to know how my known common ancestors are grouped.  So I split my matches up into groups and counted them manually (whew!):

cM Group
Number with
Common Ancestors
Number of
DNA Matches
> 70

I didn't take the time to count the number of AncestryDNA matches in groups below cM = 21.

So the DNA Matches with a known Common Ancestor are spread through the cM ranges, but I know 29 out of my top 60 AncestryDNA matches.  For most of those that don't have a known Common Ancestor, the Shared Matches can help identify a probable family line.

For "Common Ancestors" with 20 cM and above, and for some with less, I have added the known ThruLine for my DNA match to my RootsMagic database, and have added a custom "DNA Match" Event to the DNA match tester, which looks like this:

"xxxxxxxxx is an AncestryDNA match with Randall Seaver, sharing 129 cM in 6 segments.  They are 2nd cousins 1x removed, with  common ancestors of Randall's great-grandparents Thomas Richmond and Julia White."

For the "Common Ancestor" matches that I am confident are correct (because I search for records to support each relationship), I add those persons to an "AncestryDNA" Group in RootsMagic.  Those lines are not always complete, but Ancestry Hints help - I often have much better information in my tree than the AncestryDNA tester has in their tree.

In order to try to help my CVGS DNA Interest Group colleagues work with their DNA Matches, I chose to use the Leeds Method to determine which grandparent line my top AncestryDNA matches were in.  I used a spreadsheet, and added more columns to help me sort things out.  Here are my top 34 AncestryDNA matches (with names anonymized), which is 50 cM or more:

As you can see, I added the known relationship and the known common ancestors (or the probable ancestor line based on Shared Matches) to the chart.  I will soon add a column for genetic cluster information.  What other column should I add to this spreadsheet to make it more useful?  I've considered breaking it down further so I have 8 columns for the 8 great-grandparents, but haven't done that yet.

The CVGS group thought this chart might be helpful, so I sent them links articles about the Leeds Method, and my chart above, for their edification and encouragement.  Perhaps it will help them sort out their DNA Matches.  In the group meeting last week, they all had AncestryDNA matches, but were unsure what to do with them.


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Louis Kessler said...

That's a very interesting analysis, Randy. Thanks for showing what a large American family tree provides with respect to DNA matches. Compared to your 325 common ancestors, I only have 7 despite having many more matches due to endogamy.

In my >70 cM group, I have just 3 common ancestors out of 32 DNA matches.
And from 21-70 cM, I have just 4 more out of 16,482 DNA matches.
I have no common ancestors under 21 cM despite my having 156,969 distant DNA matches.

Dara said...

And, with a typical Irish family tree, I have only 24 Common Ancestors, 15 of whom share 21cM or more, out of 31,933 matches. 4 of 6 matches above 70cM have Common Ancestors and 2 of 13 share above 40cM.

mbm1311 said...

My tree is almost all Irish. I manage 12 tests and have not broken down my numbers but have an aggregate of over 100.

Greenhill39 said...

I have some false common Ancestors. As the trees get large and the shared cM shrinks it is possible that the common ancestor in the tree is not the DNA match. In one case the common tree ancestor is on my father's side but the DNA match shows shared matches on my mother's side. It is very possible that some of my DNA matches may have a common ancestor on both my Mother's side and father's side but my mother has colonial America roots and my father's ancestors are 19th and 20th century Irish/English immigrants.

Mel said...

I decided to compare my results to yours. Although I can take my tree backwards many generations, most of my matches are going to come from outside the US. My Common Ancestor results come nowhere near yours.