Hello,
I've investigated your problem. It was caused by the fact that entity
http://mouse.owl#genid1217 is never declared. It was thus replaced by None.
I've fixed that in the development version of Owlready: undeclared entities are now replaced by their IRI (I cannot create an individual for them because they have no associated ontology).
So you can now do:
>>> from owlready2 import *
>>> onto = get_ontology("file:///tmp/mouse.owl").load()
>>> onto.MA_0000004.hasRelatedSynonym
['
http://test.org/test.owl#genid1217']
Best regards,
Jean-Baptiste Lamy
MCF HDR, Laboratoire LIMICS, Université Paris 13