The scenerio described in the question is blocking effect. Blocking effect According to Kamin can be regarded as effect of conditioning of an association that exist between two different stimuli, i.e a conditioned stimulus and unconditioned stimulus which is been impaired if, at the process of conditioning, conditioned stimulus is been presented along with unconditioned stimulus which has been connected along with the unconditioned stimulus. It explained that blocking of conditioning to a stimulus is possible provided that the stimulus is been reinforced in conjunction along with previously conditioned stimulus.
In the trp (tryptophan) operon, the* repressor protein *binds to the product of the trp operon synthesis pathway, called *tryptophan *forming the active *corepresser *
