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.