Re: DNA match to me but not my parents #dna

David Ellis

Lynn Pollak Golumbic wrote:

During the last year my parents (who are 3rd cousins) and I did the
autosomal family finder test on FTDNA. One surprise was that a very good
friend and neighbor with whom I thought I had no family relationship came up
as a 5th or remote cousin on my test, with 116 Shared Cm and a longest
block of 9.
He did not show up on either of my parents' tests. Can someone explain why?
I am not adopted, I show up as a daughter of each of my parents.

This is a consequence of Ashkenazi Jewish endogamy, the fact that we married
almost exclusively within our own population. We are all descended >from a
"founder" group of approximately 350 people who lived some 700 years ago.
For quite a few generations, Ashkenazi Jews married their first, second and
third cousins before the population's rapid growth in the 18th and 19th
centuries. As a consequence, we share lots of little bits and pieces of DNA
from way back. These common bits skew the autosomal analysis algorithms
because they look just like the shared DNA segments that signify
genealogically accessible relationships within the most recent four to five
generations. As a result, the majority of relationship estimates make the
common ancestors appear to be much closer than they actually are. We may be
projected as fourth cousins, but in reality we may be eighth to twentieth
cousins many times over because of the repeated endogamous marriages.

People listed in the results as fifth to distant cousins are almost always
very distant, out of reach of conventional genealogical research. There is
a "tell", however. The general rule of thumb that I've found most useful
(although not foolproof) is to investigate a match further if it contains
one segment of at least 20 cM and another segment of at least 10 cM.
Matches that don't meet this criterion are unlikely to yield an accessible
relationship. A long segment of only 9 cM is almost certainly a sign of no
common ancestor within reach. For matches that do pass this test, you may
or may not be able to connect your family trees, depending on how far back
your paper trail goes. I have one match with a 56 cM segment and a 20 cM
segment that I'm convinced is a third cousin of my dad, but we cannot
confirm such a relationship because I've been able to trace siblings of only
nine of my sixteen fourth generation ancestors, while my match doesn't trace
as far back.

Keep searching, and focus on matches that look promising. I have over
19,000 DNA matches on FTDNA, and I've been able to find some distant
cousins among them. I hope you'll enjoy similar successes.

David J Ellis
Natick, MA

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