Identifying the inheritance pattern
Short answer

As said @Biogirl, "This pedigree can not lead to one absolute answer". We miss information in order to answer this question.

Long answer

Should we make the assumption that the allele causing the disease (A) necessarily come from the lineage you draw or can it come from the partners of the second generation (offspring of GrandParents)?

Then, depending on your answer to this question...

If yes (= if the allele causing (A) comes from the GrandParents):


This genealogy is impossible if the type of inheritance is as simple as you suggested (either recessive or dominant, no epigenetics, no epistasis, no effect of the environment, no protein-mediated disease via the vomb,... even if it can be autosomal, sexual or mitochondrial genes) 


If no (= if the allele causing (A) do not come from the GrandParents only)


Everything is possible (from the space of possibilities that you suggested)

some possibilities would be more probable than others (depending especially on the allele frequencies)