Utilizing the DNA of a Close Cousin, Part Two

The arrival of my mother's maternal first cousin at 23andMe enables me to more definitely categorize our genetic cousins into branches of the tree using autosomal DNA.

Of particular concern was the beginning of chromosome 15.  My mother has several matches that share a long segment with her from the beginning, zero, through 24,000,000.  These genetic cousins are usually from Latvia or at least one parent was.  I figured that these people were from my mother's father's side of the family, as my maternal grandmother has no Latvian ancestry known to me.  But we have to keep an open mind about our notions of our origins when we analyze our DNA.



Jody's mom versus JZ, TD, and dg.
Question posed by this match:  Do these cousins match through the paternal or maternal side?

When my mother's brother tested, he did not match any of these people.  In addition, he and my mother did not match on chromosome 15 until the 24,000,000 mark.  This is because you have two sides to your chromosomes:  one is from mom, the other from dad.  My mother and uncle did not inherit the same piece of the beginning of chromosome 15 from their mother.

So I could not be sure if these people matched my mother through her father or her mother.

Then along came the maternal first cousin, F.O.  He and my mother match at the beginning of chromosome 15, thus revealing my mother's maternal side of this segment. 

Isn't it amazing that in all of these people, the chromosome "broke" in the same spot?

Comparing F.O. to the other people who match my mother at the beginning of chromosome 15, we find that F.O. does not match these people.  So we can conclude that this group of Latvian cousins matches my mother through her father and not her mother.  Now to find the elusive common ancestor of all of them . . .

Maternal first cousin F.O. versus the Latvian matches.

Utilizing the DNA of a Close Cousin, Part One

Having my mother's maternal first cousin ("FO") in the database at 23andMe helps me isolate the matches into my mother's maternal side or paternal side.  My mother's uterine brother ("AH") is also in the database.  There are areas in the DNA of both siblings from their mother, but are not shared between them.  One such instance is with my father's third cousin, RS.  RS matches my mother but not her brother, so we cannot be certain if the match is through the maternal Irish side or the paternal side.

Fortunately, FO matches both RS and my mother on the same segment on chromosome 18.  As FO is a maternal first cousin, this indicates that RS is related to my mother through her mother, even though RS does not share any DNA with my mother's brother.

23andMe

If we travel back in my maternal grandmother's tree and back in some of the Irish branches of RS's tree, we will start to see the same ancestors.

Close Cousin: Percentages

Two people who are related closer than third cousins will share a percentage of identical DNA within a predictable range.  My new match at 23andMe, my maternal first cousin once removed, fits this predicted percentage.

International Society of Genetic Genealogy
Wiki

My mother and her brother both share 11.9% identical DNA with their first cousin.  As the next generation from these three, my share of the identical DNA should be cut in half.  My mother's cousin and I match 5%.  This percentage is on the lower end of the predictable amount shared.  We should be grateful for this because the lower percentage bumped us into a second cousin prediction and allowed him to be revealed in my account as a match, enabling me to reach out to him.

For illustration purposes, my father and his first cousin once removed match at 6.71%.  I match this first cousin twice removed at 4.44%, which is just short of the 5% I share with my first cousin once removed.  That is how random DNA inheritance works.  You end up carrying more DNA from some ancestral lines than others.