The phrase "mutant phenotype" refers to the observable characteristics or traits that are different from the norm in an organism due to a genetic mutation. Basically, it describes the physical changes or traits caused by a mutation. Full definition
Phenotype annotations for a gene are curated single mutant phenotypes that require an observable (e.g., «cell shape»), a qualifier (e.g., «abnormal»), a mutant type (e.g., null), strain background, and a reference. (yeastgenome.org)
We identified mutant phenotypes for 1,722 genes, about two - thirds of which were not previously associated with a phenotype. (sanger.ac.uk)
Expressing GLKs increased the chlorophyll content of fruit, whereas SlGLK2 suppression recapitulated the u mutant phenotype. (science.sciencemag.org)