DNA Matching with a Third Cousin Revisited

DNA results are arriving for multiple family members who recently tested at 23andMe.  Processing and analyzing them is time-consuming.  Challenge Accepted.

First up for discussion is one of the first people to submit his DNA, my father's third cousin.  (Happy Birthday!)  Third cousins share a pair of great great grandparents; in this situation, Calvin Cook and Mary Neal, both born around 1830 in Morris County, New Jersey.  Once we reach the third cousin level of relation, the chances of sharing any identical DNA begin to decrease.  If third cousins do match, the percentage will be around 0.78% (watch the decimal) over two to six segments.

International Society of Genetic Genealogy

We lucked out and my father and his third cousin match within the predicted range:  0.93% over two segments.

23andMe autosomal DNA comparison between third cousins.
They share identical DNA on chromosomes 13 and 21.

The value in this information is uncovering which segments in the genome of my father and our third cousin could be attributed to Calvin Cook or Mary Neal.  As more people submit their DNA and compare their genome to ours, we can find matches.

23andMe autosomal DNA comparison
of two known third cousins
against two others who match both of them on the same identical area

These distant cousins match both my father and his third cousin on the same segment of their DNA.  This predicts that if we travel backwards in all of our family trees, we will find an identical branch, or the Most Recent Common Ancestor.

Finding the elusive Most Recent Common Ancestor is not easy.  Most distant cousins in the database do not have extensive family trees.

My father's three siblings tested their DNA.  Fortunately, all three also match their third cousin- with more DNA.  The amount of matching varies, but is still within a third cousin range.

23andMe autosomal DNA comparison
Relative Finder/DNA Relatives function
Four sets of known third cousins

23andMe autosomal comparison
Three siblings versus known third cousin.
The segments circled in red are segments newly identified as Cook/Neal genetic material.
All three siblings provide more matching DNA than the original comparison with my father.

My father's siblings share the two segments shared between my father and their third cousin, in addition to several other segments (marked in red in the screenshot above).  I can now identify these additional segments as Cook/Neal DNA in my father's siblings and their third cousin.  This enables me to identify more people in the database who share ancestry within our specific Cook/Neal branches.  This is especially important because Mary Neal is a "brick wall" in our family tree:  We do not have her parentage.  I suspect she is of Irish ancestry and am not surprised to find that most matches in common among my father, his siblings, and their third cousin are in Ireland.

Next, we consider the DNA shared between this third cousin and the next generation:  My sister and me.  Our relationship is called Third Cousins, Once Removed.

23andMe autosomal DNA comparison
between two siblings and known third cousin, once removed

I share no DNA with my third cousin, once removed, while my sister shares one small segment.  This is entirely possible and demonstrates how little DNA, if at all, you share with distant relatives.

Now here is where you can and will run into trouble with your DNA comparisons.

Let's say that my parents and their siblings were not available to test.  This is a very real situation for most of you reading this.  After finding no shared DNA between me and this third cousin, once removed, we might question our records and wonder if we had a non-paternal event in our lines.  Next my sister tests and are relieved and thrilled to find some shared DNA.  Looking at my sister's small segment shared with this third cousin, once removed, we would incorrectly conclude that this little segment is Cook and/or Neal DNA.  The segment shared between my sister and our third cousin is not from our father, but rather from our mother and is likely not Cook or Neal DNA.  (Leave open the possibility that our mother is related to Mary Neal, our father's great great grandmother.)

23andMe autosomal DNA comparison
Known paternal third cousin versus father/mother/child trio.
The green segments represent third cousin matching.
The blue segments represent a match to a distant cousin, passed on to the next generation.

Fascinating, isn't it?  Our father's cousin is also our mother's cousin!  Our father shares two large segments of DNA with his third cousin and we inherited none of it.  The small segment shared between my mother and my father's third cousin is inherited by my sister only.

Testing more cousins on our mother's side should help assign this double cousin to a maternal branch as well.  (Hint hint.)