Humphrys genealogy

Genealogy research by Mark Humphrys.

Blennerhassett problem - DNA testing - CASH



DNA testing - inside the Cashel tree.
For background see DNA testing for the Blennerhassett problem.

As part of the testing, we proved many lines inside the known Cashel tree.



We compare individuals from the groups defined here.

This page will compare:


Proven cousins

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

Here are the known cousin relationships.
All the people here descend from George Cashel (born 1807).

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 compare these known relations within the Cashel family.

Note that we get false negatives:

The DNA positives here make sense and match the family tree. It clearly show the two groups:

Much of the Cashel tree is now DNA-proved:
  • Between these matches on Gedmatch, and other matches on Ancestry, much of the Cashel tree is now DNA-proved.
  • Search for DNA proven lines in my tree (see the Cashel section).

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:

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

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