A weighted graph G ω consists of a simple graph G with a weight ω, which is a mapping, ω: E(G) → Z\{0}. A signed graph is a graph whose edges are labeled with −1 or 1. In this paper, we characterize graphs which have a sign such that their signed adjacency matrix has full rank, and graphs which have a weight such that their weighted adjacency matrix does not have full rank. We show that for any arbitrary simple graph G, there is a sign σ so that G σ has full rank if and only if G has a {1, 2}-factor. We also show that for a graph G, there is a weight ω so that G ω does not have full rank if and only if G has at least two {1, 2}-factors.