Wednesday, February 12, 2014

Genetic Genealogy: Fourth Cousins

Another successful match in my genetic genealogy pursuits at 23andMe!  This time we have my father's fourth cousin.  The most recent common ancestors were my father's great great great grandparents:  Eliakim Marsh and Susan Long.  They were born in the 1810s and lived in Elizabeth, New Jersey.  [Elizabeth is currently in Union County, but the area was Essex County until 1857.]  Eliakim died in 1881 and Susan in 1882.
With an autosomal DNA test, you have slightly less than a 1 in 2 chance of sharing any identical DNA with a fourth cousin.  My father and his three siblings all share DNA with this fourth cousin.  The amount of DNA shared ranges from 0.66% to 1.18%, which is on the high side for a fourth cousin match.  This could indicate that we are related on more than one line, or that Eliakim and Susan were related to each other.  Additional research will yield more information.

Of special note is a match to my aunt and uncle on the X chromosome.  This is a 23 cM segment from either Eliakim or Susan- we cannot tell which one at this point.  The X chromosome has a specific inheritance pattern.  The chain is broken in any father to son descent.  A father passes on his only X chromosome to a daughter- an exact copy.  A mother has two X chromosomes that are recombined, likely into two or three segments, and passed on to her children.  Thus, large segments on the X chromosome may travel intact for many more generations than autosomal segments (the other 22 chromosomes).

By identifying the ancestors responsible for an area of a chromosome, we can specifically use that one branch as we look at the other DNA cousins who match on this same segment.

The path of inheritance for this segment of the X chromosome for my aunt and uncle was:
1.  Eliakim Marsh or Susan Long
2.  Susan Marsh
3.  Minnie Bishop
4.  Eugene Cook
5.  Beulah Cook
6.  Jody's aunt and uncle

How did we figure out the relationship and the most recent common ancestors?  Geography.  We looked in our family trees and identified people living in the same area of the world at the same time.  New Jersey, United States, 1800s.  Then we compared surnames.  In reviewing my notes on Marsh and Long, I realized that we had first corresponded years ago on these same people, where the relation had already been figured out.  Here we are, connecting again, because we share identical DNA from our common ancestors born 200 years ago.