Humphrys genealogy

Genealogy research by Mark Humphrys.

Blennerhassett problem - DNA testing - LETITIA



DNA testing - inside 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 say that the Letitia theory is not true. However the results on this page are still good.
As part of the testing, we proved many lines inside the known Letitia Blennerhassett tree.


We compare individuals from the groups defined here.

This page will compare:


Proven cousins

First, we lay out the known cousin relationships (known from the family tree) of people inside this branch.
Then we can compare DNA results to their known cousin relationship.

Inside LETITIA, we have only done cousin numbers for ED and REV.
Everyone here is proven related. Common ancestor is Edward Blennerhassett of Rossbeigh (born 1705).

Total segment matches (minimum segment 9)

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

We include male-line and female-line relations of Letitia Blennerhassett of Tarbert.
So not everybody is related to everybody.

We get false negatives:

The DNA positives make sense and match the family tree:

Quite a bit of the Blennerhassett tree is now DNA-proved:
  • The links inside the two ED groups and inside the REV group prove a lot of the Blennerhassett tree.
  • There are also links across those 3 groups (REV and the two ED groups). The strongest match is 16.2 cM and there are other significant matches. The 16.2 cM match is between two people who are 6th cousins once removed.
  • DNA is connecting the descendants of 3 different children of Edward Blennerhassett (born 1705). DNA is confirming the Edward Blennerhassett descent.
  • Between these matches on Gedmatch, and other matches on Ancestry, quite a bit of the Blennerhassett tree below "Black Jack" is now DNA-proved.
  • Search for DNA proven lines in my tree (see the Blennerhassett section).

GOD and MUL are not related to ED, but they are related to REV.


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.


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.

Here is the known cousins table again (for ED and REV):

The Gedmatch estimated MRCAs are a fairly good fit to the family tree. There are some false negatives.

This seems DNA-proven:


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