Wednesday, June 20, 2012

DNA Matching: Autosomal versus Y-DNA

When my DNA results produce matches at 23andMe, I look at two factors to try to figure out the most recent common ancestor, or the source of the little identical segments of DNA.  The first source is common surnames and the second is common geographical areas.  Both options pose their own problems, but you need to try some path to make the match.  Surnames change over the generations.  People move around, from state to state and continent to continent.  Someone may not uncover all areas that their ancestor called home.  Autosomal DNA testing reveals the little segments of DNA that you still carry from your distant ancestors.  Some other descendants of these same ancestors still carry the same segments, producing a "match."  The trick is identifying which ancestor or ancestors you have in common with your genetic match.

I have a few matches where I can't say for sure that we have uncovered the common ancestor, but we seem to have good leads.  One is the surname Rockefeller.  Henry Oscar Rockefeller compiled a family genealogy in the early 1900s, focusing mainly on this line, the immigrant Diell Rockefeller (died New York 1769), but also discussed some of the other Rockefeller lines in the area.  I descend from Diell Rockefeller through my father's mother.  At 23andMe, we both match a man who is descended from another immigrant, Johann Peter Rockefeller (died New Jersey about 1766).

The Transactions of the Rockefeller Family Association by Henry Oscar Rockefeller, 1910.
[Viewable through Google Books or Ancestry.com]
A line descended from Diell Rockefeller merged with a line descended from Johann Peter Rockefeller.
My line descends from Diell's son Simon [red] through another daughter, Margaret.
Our genetic "Distant Cousin" descends from Johann Peter's son Peter [blue] through another son, Henry.

23andMe.com
Autosomal DNA comparison of the descendant ("Distant Cousin") of Johann Peter Rockefeller to my father and me.
We match on a small segment of Chromosome 12.

It is not known if Johann Peter and Diell were related.  Does the above DNA comparison show that the two Rockefeller immigrants Johann Peter and Diell were related?  Not necessarily.  We could have other ancestors in common that we do not know about that are causing the match.

The DNA testing that could show if both Rockefeller immigrants were related is Y-DNA testing.  Direct male descendants of both men could compare their Y chromosomes.  The Y chromosome is passed almost unchanged from father to son and then to his son as so on down the line of descent.  Neither my father nor our "Distant Cousin" here can participate in such a test because the last Rockefeller in both of their lines was a female, hence breaking the Y chromosome inheritance.