DNA Research #DNA DNA percentages on Gedmatch #dna


Martin Davis (com)
 

David Goldman wrote: "Although my basic DNA report on FTDNA and Ancestry
record being 100% Ashkenazi, this appears different according to the
reports appearing in Ged Match using Admixture Heritage, the project
Eurogenes through JTest and Eurogenes 13. I don't understand this. But
maybe the experts can explain it to us in layman's languages. It includes
the largest percentages (25-30% each) for "Ashkenazi," East European,
West Mediterranean, East Mediterranean, and others, even 2% for West
African and East African. I guess one could contemplate migration
patterns involving converts along the way, including slaves way back when
from this data (which would be great for a historical novel). The Jtest
also brings up "Amerindian," Bulgaria, Romania. How does one put this all
into some kind of perspective beyond the simple category of Ashkenazi Jew?"

Each of the commercial companies has its own algorithm for calculating
genetic heritage via autosomal DNA. Most base this on a combination of
reputable academic research and the individual companies own research; the
latter using their own database of kits registered with their company. The
influence of the database seems to vary >from company to company. There are
obvious flaws in using a 'local' database - the obvious one being the
accurate identification of origins of the particular segments of DNA. This
is as much an issue of policy as it is of science.

As a pertinent example, although Ashkenazi Jews have been an identifiable
entity for around 1000 years, the actual fine detail of the genetic
admixture of 'Ashkenazim' has changed over time - for instance with the
migration (and total absorption) of some Sephardim into Ashkenazi
communities of northern and central Europe. Using FTDNA calculations, that
flaw can be obvious - with the results of the FTDNA myOrigins calculator
showing 100% Ashkenazi origin but FTDNA's Family Finder database
identifying many linked, but often distant, non-Ashkenazi kits. The 'why'
of that seems to be that FTDNA did not develop a 'deep ancestry' calculator
(say one which would show heritage >from 500 years plus - which would have
diminishing accuracy) but developed one with samples which would be
'guaranteed' correct - that is one where all four grandparents are/were
identified as >from known Ashkenazi origins; rather than more distant and
speculative indicators of earlier origins.

The Gedmatch calculators, which have their own quirks and inaccuracies,
normally use a range of academic samples to build a calculator to analyse
the data and to come up with a picture for the targeted groups for which
the calculator is intended. So for instance MDLP is a Gedmatch based
global calculator which attempts to break down results into different
parts of the world. To quote >from the Genealogical Musings blog, "It's
good as an overview, but if, for example, if you already know you're
European, it's probably unnecessary. It's also heavy on ancient groups."-
http://genealogical-musings.blogspot.com/2017/04/finally-gedmatch-admixture-guide.html .
The Eurogenes calculators tend to be the go-to calculators for people of
European origin but the JTest has been virtually disowned by its creator
Davidski, " Let me reiterate that this test was only supposed to be a fun
experiment. It was never meant to be the definitive online Ashkenazi
ancestry test. And even as fun experiments with Admixture go, it's now
horribly outdated, and probably useless for anyone with less than 15-20%
Ashkenazi ancestry." See
http://bga101.blogspot.com/2012/09/eurogenes-ashkenazim-ancestry-test-files.html
for the full blog.

David asked "How does one put this all into some kind of perspective
beyond the simple category of Ashkenazi Jew?". The most straightforward
answer to that question is that one needs to familiarise yourself with
the calculators, what they are intended to do and their results for your
kit. Unless one wants to pay a professional, there is no avoiding that
hard work, which includes reading through the guidance material which
Gedmatch link to their home page ('DNA for Dummies') and also doing the
same with material provided by the commercial company of your choice.

Martin Davis (London - UK)

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