Tuesday, December 1, 2020

Dear Randy: Why Do I Have So Few Known Common Ancestors in My DNA Matches?

 A society colleague asked me this question recently, and I've been thinking about it for awhile.

1)  Here is my response:

*  The main reason is that most of your DNA matches don't have big enough online family trees that might show a common ancestor.  And many of your matches have no tree at all.

*  Another reason is that the family trees your matches have may be wrong - they may have added the wrong parents to their tree due to bad information or bad decisions, or they may have one or more NPE (not parent expected) due to adoption or out-of-wedlock child birth.  

*  One more reason - your own family tree may be wrong!  I may have one or more NPE events in my ancestry and not know it, or may have made a bad decision on the parents of an ancestor.

2)  Examine your DNA matches:

*  I have 34,475 DNA matches on AncestryDNA, with 1,537 matches with 20 centiMorgans (cM) or more.  All of those above 20 cM should be real DNA cousins according to the experts, unless there is endogamy in my ancestry (of which I have little known, if any).   I have about 410 ThruLines (where Ancestry shows me a common ancestor and the line to me and the line to my DNA match), so about 1.2% of my total matches.  Some of those ThruLine matches share less than 20 cM with me, but my paper trail and the DNA match's paper trail shows a common ancestor.  

*  It's even worse on MyHeritage - I have 8,949 DNA matches there, and only 8 "Theory of Family Relativity" matches.  These also show a known common ancestor based on my tree, the match's tree, and other trees or records.  So that's only 0.1% of my total matches.

*  However, even those ThruLines and Theory of Family Relativity lines may not be correct - I have several "Potential Ancestors" on my ThruLines which don't make much sense, but there is an Ancestry tree out there that Ancestry used to create it.  Or my research is wrong...it could be!

*  In short - not every person who ever lived since, say, 1750 - picked because that's about the limit for the birth date of most common ancestors of my DNA matches - is in an online family tree.  There are over 7.5 billion living people now, and almost all of them are not in anyone's public online tree because of privacy reasons.  Not every person who ever died is in an online family tree - I find persons in my research who are not in any other online family tree until I add them.

So that's WHY. 

3)  Now - what can you do about it?  

*  According to the Shared cM Tool on DNAPainter,  229 cM is the average match value for a second cousin (common great-grandparents), with a range of 41 to 592 cM.  73 cM is the average match for a third cousin (common second great-grandparents, with a range of 0 to 234.  35 cM is the average match for a 4th cousin (common third great-grandparents), with a range of 0 to 139.  Some other relationships with a once- or twice-removed cousin relationship, or with a half-cousin relationship, changes the cM numbers.  Not every match has cM numbers consistent with the average, some have more, some have less, and by the time you get down to 3rd cousins not every 3rd cousin is your DNA match because of the randomness of DNA recombination generation by generation.

*  There are some possibilities, especially for matches with more than 20 cM.  For those with significant tree people (say 100 profiles or more), you can search their trees for known surnames - Ancestry does it for you.  If you're lucky, their tree might get back to 4th or 5th great-grandparents, or provide enough information for you to try to go back one or two more generations.  

*  You can try to contact your DNA Match with no tree or a small tree and see if they have a more fully-leafed tree that is not on Ancestry or MyHeritage.  You can use the message systems on Ancestry and MyHeritage.  My experience is that most of the Matches do not respond.  The ones that might respond are those that have logged into Ancestry or MyHeritage in the past week or month.  You might try to find the DNA Match on Facebook and contact them that way.

*  You can take the known ancestors of your DNA match and see if their grandparents or great-grandparents are in another Ancestry or MyHeritage tree, or in the WikiTree, Geni, or FamilySearch Family Tree with more ancestors back in time, and search for your known ancestors there. I've tried this numerous times, and have not had success.  

*  You can take the ancestors of your DNA Match that has a tiny or small tree (say with deceased grandparents or great-grandparents) and make your own tree on Ancestry or MyHeritage using their Record Hints and Matches and see if you can research your way back to one of your known ancestral families.  I have what I called a "Skeleton Tree" on Ancestry that I use for this type of analysis - it includes only ancestors, and is private and not searchable.  I add my DNA Match name and their known ancestors.  Other researchers have called this type of analysis a "Mirror tree," a "Quick-and-Dirty tree,"or a "Floater tree."  I've had some success with this - I have found a common ancestor for about 30 to 40% of the DNA matches that I've used it for.

*  You can add descendants of your 4th and 5th great-grandparents to your tree that is connected to your DNA test.  By extending your tree down to more recent times, you may find - and Ancestry and MyHeritage may find using their ThruLines and Theory of Family Relativity technology - the grandparents or great-grandparents of your DNA matches.  Every week I add more descendants of my 4th great-grandparents to my family tree and upload them to Ancestry Member Tree.

*  Some of your DNA matches may "cluster" together using Genetic Affairs or a similar technology.  Unfortunately, this doesn't work any longer for AncestryDNA, so you will have to use "Shared Matches" there,  There may be trees in that cluster that have an obvious connection with a specific surname in a specific place.  You could research, say, a 4th great-grandparent from one of those trees for descendants and perhaps find one of your ancestors in the process.  

4)  I'm sure there are other techniques that people have used, and would be interested in hearing about them.


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Copyright (c) 2020, Randall J. Seaver

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1 comment:

Diane Gould Hall said...

What an excellent explanation Randy. You made a couple of points I hadn’t thought of, so thank you for that. Overall, a very thorough and easy to understand answer to a somewhat difficult question.