Thilo Sarrazin and the Question of Population Genetics
Thilo Sarrazin, a German Central Banker, said not that long ago that "all Jews share the same gene".
This is interesting semantic territory because, of course, if you want to get technical there are innumerable gene variants shared by all Jews.
It just is that these gene variants are probably also found in all Humans.
Now with the Ashkenazi Jews, they might be inbred enough that there's a gene where it somehow managed to reach a rate of 100% in Ashkenazis, while not reaching 100% in non-Ashkenazis.
But it would be a situation where the Ashkenazis had a rate of 100%, while other Caucasoids had it at 90% or thereabouts.
(Note that even a 10% difference between Ashkenazis and another Caucasoid group on a particular gene variant would be unusual. For example, even looking at the non-isolated European population most dissimilar to Ashkenazis, Russians, the average gene variant was found to have a 1.37% difference in the rate found in Russians as compared to the rate found in Ashkenazis.)
The genetic distance between Ashkenazis and other Caucasoids, especially Caucasoids descended from one or the other of the Ashkenazis' parent populations, is small enough that any process which put a gene variant at a 100% rate in Ashkenazis would also have put it at a very high rate in other Caucasoids.
What makes Ashkenazis distinct from other Caucasoid populations is the tens of thousands upon tens of thousands of gene variants which are found a little bit more in Ashkenazis, and a little bit less in other Caucasoids.
It has not yet been determined if all full-blooded Ashkenazis are always more related, in terms of the number of identical base pairs, to the average full-blooded Ashkenazi than to the average member of non-Ashkenazi groups.
But it seems probable that this is the case.
The thing is that Ashkenazis have a large Middle Eastern component to their genetics which pulls them further away from Europeans than you'd think based on how long they've been in Europe.
At the same time, they have a large European component to their genetics which pulls them away from Middle Eastern groups.
Anyway, I suppose part of why some took offense at Sarrazin's comment is because it made Jews seem more genetically distinct from others than they actually are.
Even if a gene exists which is fixed in Jews, while not being fixed on the species level, this gene would certainly be found in millions of non-Jews, and possibly billions of non-Jews.
It's large scale combinations of genes found more in Jews than non-Jews which make the Jews genetically distinct from non-Jewish populations.

