Again I’m looking for an illustrative explanation concerning the simple fact, that people with an Ashkenazim background have about ten times more genetic cousins than me. The two main reasons, cited in countless forum postings are heading both historical developments and a higher inbreeding ratio. This always sounds logical, but is nevertheless hard to understand. So here it goes with a personal visual approach!
In Cologne we have just started to explore a new excavation site in the old Jewish quarter, built in the 12th century, when the town expanded toward a muddy Rhine arm. They already found fine jewellery, burnt remains of the Bartholomew night and even a big Jewish romance of chivalry, containing a lot of erotic adventures. At that time the members of the Jewish community along the Rhine valley named them self Ashkenazim. There are thought to be about 25000 in the cities of Speyer, Worms, Mainz and Köln. Beside their ritual language Hebrew they spoke a German medieval dialect as everyday language. In the dynamics of the beginning crusades with all its hate propaganda they were pushed by west-European Christian anti-Semitism to leave their homes and move more and more eastwards, enriching their language with Czechish, Polish and Russian idioms, thus developing Yiddish.
There are almost 2000 years of Jewish presence in Europe. In 321 Caesar Diocletian confirmed a declaration from Cologne, demanding, that members of the Jewish community now should have the right to join the city government, what exactly means, that there must have been a considerable amount of Jewish citizens with enough cultural and financial power to compete in a city of about 30.000 people. The soldier camps along the roman LIMES ( including Mainz, Speyer, Worms and other) are exactly the same as mentioned to be Jewish centers later, what makes me believe, they had been here since roman times. Romans lived in the Rhine valley for about 500 years - enough time for Jewish merchants and families to find a way up north.
While the Ashkenazim were forced to emigrate eastward, parts of their Sephardic distant cousins joined them, leaving Spain and Portugal. Later on others became the first to cross the ocean for the Americas. They reached New Amsterdam via Brazil in 1654. This first immigration wave was followed by a German one after 1840, before the main wave started around 1880, bringing until 1924 more than 2500000 eastern Jews into the US. This seems to be the wave, whose genetic echo we encounter nowadays.
But this alone cannot explain the amount of matches, so I wanted to have a closer look on what is called the “inbreeding ratio”. Traditional genealogy already gave definitions, explaining the simple fact, that our binary ancestor tree (doubling in each generation) would produce an absurd situation, outnumbering the world population after enough generations. The solution for this paradox is called implex or pedigree collapse or Ahnenschwund. It means that further back in time some ancestors must appear in several positions, that there must have been more and more cases of cousin or uncle-niece marriages. I do not want to focus on the different reasons for this phenomenon, but head for its principal genetic implications, which are to illustrate. The usual pedigree tree gives us eight great-grandparents, thus shrinking the genetic partition of each to about 12,5 % of a great-grandson or daughter after three generations.
This is what it looks like in the light of a cousin or uncle-niece marriage. As one can easily see, a cousin marriage reduces the number of great-grandparents to six and produces two generations of same percentage, while marrying an uncle only needs four great-grandparents.