The Mouse Genome Informatics (MGI) Phenotype ontology  contains data about mouse genotype - phenotype associations primarily obtained by curators from the literature.
Phenotypic terms are related by a directed acyclic graph, the Mammalian Phenotype Ontology .
Statistics can now be found in the footers of each table in the GREAT web output.
All ontology data was downloaded and built between Oct 14, 2011 and Feb 18, 2012.
GREAT v1.8 Mouse Phenotype statistics are available here.
GREAT v1.2 Mouse Phenotype statistics are available here.
Blake, J.A., et al. The Mouse Genome Database genotypes::phenotypes, Nucleic Acids Res. 2009 Jan; 37(Database issue):D712-9.