Figure 1

An example of a directed acyclic graph (DAG) taken from the gene ontology (GO). The solid arrows indicate the GO 'part-of' link and the dashed arrows the GO 'is-a' link. The GO unique identifiers (IDs) are printed below each term. The term 'Cell Differentiation' has two parents (Cellular Process and Development), which in turn link back to the same antecedent 'Biological Process' which is part-of the Gene Ontology. The unterminated arrows leading from Cell Differentiation indicate that it has a number of offspring terms.