FF Chromosome Browser #dna


sbloom@...
 

I, similarly to many on JewishGen, have attempted to wrap my head
around the meaning of my Family Finder tests and the significance of
some of the results. Since almost none of us are aware of the nature
of or criteria involved in the algorithm being used to define our
cousinhood we can only guess at the basis of these conclusions.
We don't know the exact algorithm, but many of us are aware that
certain key factors are weighted highly: longest block, number of
shared blocks above a certain threshhold, and total cM of blocks shared.
You might even be able to reverse engineer the algorithm by looking at
these data for everyone you match. Its doubtful you'll get something
exact, but it might be close enough to see what goes into the
calculation.

This weekend I decided to use FtDNA's chromosome browser to compare
my 22 third cousin hits and found some, to me, intriguing results. I
found 3 people who share almost exactly with me an apparently
significant stretch of chromosome 2. All three of these show the
exact same similarity endpoint. Two of those same people appear to
share exactly the same 16.81 cM region with me. Now, I realize that
these kinds of results could be biased to a certain extent by such
things as recombination "hot spots" or perhaps in this case "cold
spots". I can see also how these biases could be caused by
unevenness of the distributions of the chosen SNPs thoughout the
genome, but I find it hard to see how two people could share such a
substantial region so exactly without being related. Any thoughts on
these observation? Thanks in advance.
I've seen many people lately looking into these shared blocks (that
is, seeing if everybody who shares the same block on Chromosome 5 is
related somehow, etc.). I know its becoming more common to do this
in some surname projects, especially when it seem they don't relate
by YDNA (so, then, the question becomes, do they relate at all?
Maybe FF and chromosome browser can add insight).

My gut feeling is that you are right to have serious concerns! I
think all sorts of bias, and, in addition, noise (for small blocks)
can be problematic. But...16.8 cM is a moderate sized block. You
probably really do have a relatively recent MRCA with these folks.
Unfortunately, "relatively recent" when it comes to DNA can still be
hundreds of years. I certainly think it would be well worth your while
(and the while of yoru matches) to explore this more via standard
genealogy. That is, push the time limit of records, or whatever you
might find for these families.

I've had a lot of doubts over this method, but probably when the
blocks are big, and the "matchees" have something that indicates a
connection (name, geography, whatever) , it might be worth exploring.

You might also try to get more of your close and far relatives to
test. That can help in determing whether these relatives are maternal
or paternal, etc..

Steven D. Bloom

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