How much of the genetics of the East Prussian Germans who
left in the mid 20th century actually came from the native Old Prussians of the region? If it was little, these people were not native to the area, and were in many cases returning home. If it was a lot, then the expulsion displaced people whose roots extended into the region since prehistory. Which is it?
I think this is an answerable question. We have Y-DNA samples of both
Old Prussian descendants and of
East Prussian Germans, as well as a good enough understanding of the Y-DNA distribution in Northern Germany to use as a baseline. I found the following percentages for the three populations of the four main haplogroups relative to one another:
Population: R1a : R1b : N : I1
OldP: 46% : 13% : 39% : 1%
EastP: 32% : 24% : 28% : 16%
NGer: 28% : 46% : 2% : 24%
[East Prussian German percentages take into account only the apparent East Prussian Germans from the Ostpreussen project]
The distribution of all four haplogroups of the East Prussian Germans predictably falls between the Old Prussians and Northern Germans. We can then average the haplogroup distances to come to an estimate of the percentage contributed by the Old Prussians. A distance can be calculated using the formula: |(NGer% - EastP%)|/(|(OldP% - EastP%)|+|NGer% - EastP%|)
We get:
Per R1a: 22%
Per R1b: 67%
Per N: 70%
Per I1: 35%
Average: 49%
So there we have it. On the Y line, the East Prussian Germans were about half Old Prussian. Since I expect the German settlers there to have contributed disproportionately to the Y line, in terms of overall contribution, I'd be comfortable in saying that the Old Prussian contribution was at least about half.