<Can anyone explain to me how there could be sharedThat occurs frequently, especially with Ancestry, because Ancestry
does not tell us the length of the longest matching string. If we knew
that length, we could eliminate many matches that are either just
noise or so far back in history that we can neither confirm or deny
their veracity. I think their theory is that they do not need to show
that information because they only show matching strips of at least 6
cMs in length, but there is a big difference between 6 cMs and the
20-30 cMs that experts suggest is significant.
Most double-sided matches result >from endogamy, the fact that
Ashkenazi Jews tended to intermarry. The marriage may have been pretty
far back in the chain, but it may nonetheless show up as a match.
My response to this is the safety-in-numbers gambit. Assume that I
have 4 cousins on my father's side and 4 cousins on my mother's side
who have tested. If the match is with only one cousin on one side, I
tend to disregard it. If the match is with 3 or 4 cousins on both
sides, one can be pretty sure that the two lines intermarried, though
not necessarily within the time when we can prove it by finding the
It seems to me that some of my cousins are very "matchable", while
others are not. I have no idea whether certain dna combinations are
more common at particular sites along the genome than others, which
would result in people who have those common combinations match more
frequently than those who lack them.