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.