Structure of Gene Ontology. Depicted here is a graphical representation of the GO term "hexose biosynthetic process" and its associated parent terms, adapted from the AmiGO website http://www.geneontology.org/ webcite. Only partial paths are shown here. The three paths branch up to higher level parent terms, leading back to the root term "biological process". Arrows between terms represent 'is-a' relationships. The hierarchy of each ontology is structured as a directed acyclic graph (DAG), with the more specific, lower level terms, having one or more direct parent terms associated with it. This is because a single child term can be defined in a number of different contexts.
Mistry and Pavlidis BMC Bioinformatics 2008 9:327 doi:10.1186/1471-2105-9-327