Final answer:
A mutant organism that synthesizes proteins constantly, regardless of environmental conditions, has 'constitutive' gene expression. This is due to mutations that affect the normal regulation of gene expression, as seen in the example of the Antennapedia mutation in fruit flies.
Step-by-step explanation:
An organism that synthesizes certain proteins all the time, regardless of environmental conditions, is described as having a constitutive expression of those genes. This means that the gene products or proteins are continuously produced, as opposed to being induced or repressed in response to specific environmental stimuli. In genetics, constitutive mutants are known to express certain genes at a constant level, which can be a result of a mutation affecting regulatory sequences or factors that would normally control gene expression. The Antennapedia mutation in Drosophila is an example where a mutant allele causes an expansion of the distribution of the gene product, leading to an abnormal phenotype.