Humphrys genealogy

Genealogy research by Mark Humphrys.

Blennerhassett problem - DNA testing - CASH v. LETITIA



DNA testing - Cashel with the Letitia branch.
For background see DNA testing for the Blennerhassett problem.

This page is about the tests done to prove or disprove the Theory of our descent from Letitia Blennerhassett of Tarbert.

The results (below) say that the Letitia theory is not true. We are not closely related to Letitia Blennerhassett of Tarbert.



We compare individuals from the groups defined here.

This page will compare:


Total segment matches (minimum segment 9)

We start by considering total segment matches, using minimum segment size = 9 cM.
9 cM is generally considered significant.

We compare our family with the Letitia Blennerhassett family.


Best match found


The best match for the Letitia theory is this 9.4 cM match of ED.5 and CASH.21.
ED.5 is in the Clifford line, descended from Mary Blennerhassett, sister of Rev. John.
CASH.21 is a descendant of Blennerhassett Cashel.


Total segment matches (minimum segment 7)

Let us try (as some sites do) reducing the minimum segment size and looking at total segment matches.

Here is total segment matches using minimum segment size = 7 cM.

Matches for CASH with LETITIA emerge:

Largest segment in match

We now take a wider look at the data by showing for every match, what is the largest segment.
Largest single segment in match, in cM.
Small segments can happen by chance. Large segments much less so.


Estimated MRCA

If there is a segment over 5 cM, Gedmatch estimates the number of generations to Most Recent Common Ancestor (MRCA).
This is an estimate from the DNA, not from the family tree.
Gedmatch can give different estimates depending on the minimum segment length you pick. We use the closest estimate, which is the estimate given under minimum segment 5 cM.

Postulated cousins

Now we consider, under the Letitia theory, the postulated cousin relations of CASH with REV and ED.

If George Cashel is the son of Letitia Blennerhassett of Tarbert, then these would be the postulated cousin relations:

Here are the Gedmatch estimated MRCAs:

There is no match with the postulated cousins. The Letitia theory looks false.

Here are the largest segments:

There is no match with the postulated cousins. The Letitia theory looks false.

Real cousins v. Postulated cousins

Let us write out the entire list of results for real cousins versus the same list for postulated cousins, separate into lists for each type of cousin, and sort the lists, to see if the postulated has the same pattern as the real.

This is where the Letitia theory really looks false. The below shows that for we should already have got some great hits for CASH. We should have a bunch of strong "blue" hits already. But we have basically nothing. The Letitia theory is false.

Here are the lists, for each type of cousin, of largest segments for real cousins versus the same list for postulated cousins (for CASH crossed with REV and ED).
We start at "4.0" cousins (there are no closer living postulated cousins).

The Letitia theory looks false.
5.0 and 5.1 cousins should have much stronger matches.

Conclusion - The Letitia theory looks false

We have enough samples now to declare that the Letitia theory looks false.

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