Is a

From HAO Wiki

This relationship is relatively intuitive. Is_a designates one class as a subclass of another class. From the relationship ontology:

id: OBO_REL:is_a
alt_id: OBO_REL:0000001
name: is_a
builtin: true
def: "For continuants: C is_a C' if and only if: given any c that 
instantiates C at a time t, c instantiates C' at t. For processes: P is_a P' 
if and only if: that given any p that instantiates P, then p instantiates P'." 
[PMID:15892874]
comment: The is_a relationship is considered axiomatic by the obo file 
format specification. In the representation of obo in OWL, where obo terms are 
represented as OWL classes, is_a is mapped on to the subClassOf axiom
exact_synonym: "is_subtype_of" []
xref: owl:subClassOf
is_reflexive: true
is_anti_symmetric: true
is_transitive: true

Using fore leg is_a leg as an example, it's important to understand a few things about the is_a relationship:

• any instance of fore leg (e.g., the fore leg of that bee under the microscope) instantiates BOTH the class fore leg AND the class leg (i.e., every fore leg you see is both a fore leg and a leg)
• the relationship fore leg is_a leg is NOT the same as leg is_a fore leg; the relationship is not symmetrical.
• This relationship always includes a child and a parent (and hence is_transitive), in that order.

Every single term should have an is_a relationship, traceable back to the root. See GO