Wednesday, June 27, 2012

Multiple cousin relationships and shared DNA: Endogamy and Consanguinity

I have read a number of posts about endogamy and shared DNA in regard
to the Ashkenazi "Jewish correction" used by 23andme and how it keeps
Ashkenazi Jews from seeing their non-Jewish cousins. Because of the
many intermarriages among European Jews the genetic relationship
predicted is often greater than that expected from the documented
genealogical relationship. These predicted higher or closer relations
push out the genetically smaller but documented higher genealogy

Many of the Ashkenazi cannot determine the relationships being shown
since the predicted range is too close and their pedigrees are not
detailed enough to make the connection. Below I'll give a non-Jewish
endogamy example where the relationships can be determined.

Last night I received a predicted 2nd cousin at 23andme.  The per cent
of shared DNA is 3.02 in 13 segments for 226cM. He (call him A) is
actually one of my multiple cousins. We (I am B) are 3rd cousins, 3rd
cousins once removed, half 3rd cousins once removed, 4th cousins, 5th
cousins, and triple 6th cousins.

Compared to my multiple 1st cousin (C) they share 10 segments for 151cM.

Compared to my and my 1st cousin's maternal double 3rd cousin once
removed (D) they share 1 segment of 0.9cM.

A is related to B and C on both maternal and paternal lines and to D
through his paternal grandmother.
B is related to A and C on both maternal and paternal lines and to D
on 2 maternal lines.
C is related to A and B on both maternal and paternal lines and to D
on 2 maternal lines.
D is related to A on one maternal line and to B and C on 2 maternal lines.

In all of these cases the predicted relationship, based on DNA shared,
was one generation closer than the documented relationship mainly due
to the high degree of endogamy and the resulting consanguinity.

Now I am waiting for A's wife's results, she is my 3rd cousin once
removed and, yes, we share multiple lines of ancestry. ;^)

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