Killer Paramecia possess toxic kappa particles (symbiotic bacteria) and depend on a dominant nuclear K allele for their maintenance.

Both cells from a conjugation between homozygous strains are heterozygous for K.

They can become Killers only if, during conjugation, they received some cytoplasm containing kappa particles, otherwise they remain sensitive.

The Killer phenotype persists only if the kappa particles are supported by at least one dominant K allele, since kk cells are sensitive even if they inherit the cytoplasm from a Killer.