Mystery DNA Cousin Demystified

A few years ago at 23andMe a close match appeared for my father, his siblings, and their third cousin, Bob, on their shared Cook/Neil line of Morris County, New Jersey.  Common ancestors are Calvin Cook (1826-1889) and Mary Neil (1830-1898).

The amount of shared DNA ranged from 1.66% with my uncle to 3.82% with cousin Bob.

The probable relation would be second to third cousin.  The variance in amount of shared DNA is within normal.  Or the higher amount could indicate that this mystery cousin is closer to Bob.

Either way, the person ignored my requests to connect through the 23andMe website.

Recently, 23andMe required users to not be anonymous.  This person bypassed this non-anonymous requirement and instead blocked sharing requests.

This person won't make or break my family tree, so I moved on.

Then Cousin Bob's cousin contacted me.  They share ancestors Patrick Bernard Brady (1830-18xx) and Elizabeth Duffy (1837-1918) of County Meath, Ireland.  They were the parents of Mary Brady (1870-1942), wife of Francis Asbury Cook (1851-1919).

She wondered how Bob was so closely related to her highest DNA match.  This Mystery DNA Cousin had limited contact with Bob's cousin and revealed Brady ancestors on two separate ancestral lines from two counties in Ireland, Cavan and Donegal.

In this situation, we are not using haplogroups to assign relationships or ancestral lines.
We are using them to confirm that we are dealing with the same elusive DNA tester.

The shared percentage with Bob, as well as the haplogroups, were the same for the Mystery Brady Cousin as for the Mystery Cook/Neil Cousin.

So if the Mystery Cousin is the same for both situations, this explains why Cousin Bob shares more DNA with the Mystery Cousin.  They are related through Cook/Neil ancestors as well as the separate Brady line.  For Bob, these lines merged in his great grandparents, Francis Cook and Mary Brady.

The above diagram is my theory on how the Mystery Cook/Neil Cousin (Mystery Brady Cousin) is related to my branch.  If this person comes forward, we can revise the this diagram if needed.