In the present paper, we introduce the generic extension graph G of a Dynkin or cyclic quiver
Q and then compare this graph with the crystal graph C for the quantized enveloping algebra associated to
Q via two maps
Q,
Q : Ω → Λ
Q induced by generic extensions and Kashiwara operators, respectively, where Λ
Q is the set of isoclasses of nilpotent representations of
Q, and Ω is the set of all words on the alphabet
I, the vertex set of
Q. We prove that, if
Q is a (finite or infinite) linear quiver, then the intersection of the fibres
Q−1 (λ) and K
Q−1 (λ) is non-empty for every λ
Λ
Q. We will also show that this non-emptyness property fails for cyclic quivers.