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Things to do for Letitia Blennerhassett - DNA testing

  
  The results so far


DNA testing of Letitia Blennerhassett

So I have a theory that George Cashel is the son of Letitia Blennerhassett of Tarbert. There is lots of paper evidence for it, but no proof.

Here is my plan to use DNA to test the theory.

  


The plan

  1. Gather as many relations of Letitia Blennerhassett and George Cashel as possible.
  2. Get their DNA on Gedmatch (where anyone can compare DNA with anyone).
  3. Do one-to-one Autosomal DNA comparison of everyone with everyone.
  4. Build up a map.
  5. See what the map says.
  6. Include control matches to see what matches you could get by chance.

  7. Do all descendants of the 8 great-grandparents of Letitia Blennerhassett of Tarbert.
  8. Do all descendants of George Cashel.
  


The 8 great-grandparents of Letitia Blennerhassett.
From [Fuller, 1910].


  

The samples

We divide our samples into groups.



  

Who should be related to who

Blue
Known relations


  BL FITZ GOD MUL ED CAPT REV CASH JOHN
BL                  
FITZ                  
GOD                  
MUL                  
ED                  
CAPT                  
REV                  
CASH                  
JOHN                  

  

The results so far

  
  

Total segment matches (minimum segment 9)

Total segment matches in centiMorgans (cM).
Using minimum segment size = 9 cM.
Different sites use different minimum segment sizes. See autosomal DNA match thresholds.



Look at REV and CASH only:


  
DNA negatives:

DNA positives:

  

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.



Look at REV and CASH only:


  

Total segment matches (minimum segment 5)

We reduce further.

Here is total segment matches using minimum segment size = 5 cM.



Look at REV and CASH only:


  

The REV and CASH matches

Here are the best REV and CASH matches so far.
  

The largest match: REV.5 and CASH.4.
The 8.9 centimorgan match (on Chromosome 3) of REV.5, a descendant of Elizabeth Blennerhassett, with CASH.4, a descendant of Blennerhassett Cashel.



The second largest match: REV.5 and CASH.3.
The 8.4 centimorgan match (on Chromosome 3) of REV.5, a descendant of Elizabeth Blennerhassett, with CASH.3, a descendant of Blennerhassett Cashel (and the uncle of CASH.4).




REV.2 and CASH.8 have smaller individual matches, but with these two matches they have the largest combined match (segments over 5) of REV and CASH.
REV.2 is a descendant of Elizabeth Blennerhassett. CASH.8 is a descendant of Edward Francis Cashel.
There is a 6.4 centimorgan match on Chromosome 15.
There is a 5.5 centimorgan match on Chromosome 18.
Combined this makes 11.9 cM.


  

Largest segment in match

A simple metric:
Largest single segment in match, in cM.
Small segments can happen by chance. Large segments much less so.



  
Look at REV and CASH only:


  

Cousins v. Largest segment

Let us analyse what largest segment we should expect for each level of cousins.
We number the cousins / relationships like this:

0.0 - siblings or parent/child
0.1 - uncle/aunt relationship
1.0 - 1st cousin (direct, no times removed).
1.1 - 1st cousin, once removed.
1.2 - 1st cousin, 2 times removed.
...
2.0 - 2nd cousin (direct, no times removed).
...


  

Proven CASH cousins

Proven cousin relations of each other inside the CASH group:



And largest segment:



The DNA tracks the family tree: Negatives do not mean much: A few 4th cousin tests should give a positive:
  

Proven REV cousins

Proven cousin relations of each other inside the REV group:



And largest segment:



  

4th cousin is about the limit

The conclusion of the above is that 4th cousin is about the limit of what DNA can do for us.
4th cousins will normally give negatives. But a few of them and you should get a clear positive hit.
Luckily, we can still get a few living 4th cousins for the Letitia Blennerhassett problem.
  

Postulated REV and CASH cousins

This chart includes the postulated cousin relations of REV and CASH.
The top left (REV-REV) and bottom right (CASH-CASH) are proven.
The REV-CASH rest of the chart is just postulated.



And largest segment:




  

Estimated MRCA

Gedmatch estimated number of generations to MRCA.



  

Cousins v. MRCA

Look at proven and postulated cousins for REV and CASH again:



And look at Gedmatch estimated MRCA for REV and CASH (estimated from DNA not from tree):



  


Conclusion - More samples needed

  

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