"Single mutants" refers to organisms or cells that have undergone a genetic change or mutation in a single gene, resulting in a specific trait or characteristic being altered or disrupted. 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)
Endoderm in toddler; lefty2 double mutants resembles toddler single mutants. (elifesciences.org)
(B) In contrast, embryos from torY9 psqrum double mutant females show terminal defects resembling the psqrum single mutant phenotype. (genetics.org)