Types of graphs. Left: simple hierarchy. Directed rule, 1 parent (e. g. NCBI taxonomy). Center: Directed Acyclic Graph. Directed Rules, but  &gt;= 1 parents (e. g. Gene Ontology). Right: Not directed graph. Nodes can be both parents and children (e. g. metabolic networks) Ref: http://www.nature.com/scitable/content/ontologies-in-biology-design-applications-and-future-77403
Own work
Toniher