History / History of two Neighbors: Poland - Germany Interrelations [283]

I'm a bit curious about this part of history between the two countries as I've heard that in the past, even during the early years of their neighboring, it wasn't uncommon for Poles to have relations with the Germans and because they've been neighbors for a long time it wasn't anything unusual to 'interbreed' (sorry for the expression). When I was young, I remember I was told by my gray-grandmother that I have a German relative. Apparently, someone in the family had married one which I guess may be taken as proof for all the above.

If we go back in time, was this actually happening and was it really common?
If so, when did these interrelations first start taking place and where does it date back to?
Does anyone have any additional information on this subject to share? (Any information is appreciated)

(Btw, I'm not against such relations as I myself am half Polish from my father and don't view it as strange).