Modern East Germany is pre-WW2 Central Germany, considering that East Germany used to be located to the east of Odra-Nysa (Oder-Neisse) line.
Moreover - at least 1/5 of inhabitants of present-day West Germany are actually East Germans who came from the east of Odra-Nysa & from the Sudeten. So modern
West Germany is also considerably Slavic (at least a dozen percent).
I wrote about this here:
http://www.anthrogenica.com/showthr...aak-et-al-2015&p=101334&viewfull=1#post101334
In the 1700s percent of R1a in West Germany was IMO as low as today in France or in the Low Countries - so a few %.
It increased vastly during the 1800s-1900s as the result of migrations from the east to the west.
Massive migration of ethnic Poles to the Ruhr region is also not a mystery. For example in the city of Dortmund in year 1910 as many as 12,2% of inhabitants had Polish as their mother tongue (in Gelsenkirchen - 17,7%; in Recklinghausen county - 15,7%; in the city of Bochum - 9,1% in year 1900). Those were immigrants and their children. Later of course they became Germanised.
Following the collapse of Yugoslavia, many dozens of thousands of Yugoslavs emigrated to West Germany.
Most of Germans that I have discussed with on internet fora say that they have some Slavic ancestors.