The word cistron is used to emphasize that molecular genes exhibit a specific behavior in a complementation test (cis-trans test); distinct positions (or loci) within a genome are cistronic.
The term cistron was coined by Seymour Benzer in an article entitled The elementary units of heredity.
He also defines replicators, more general than cistrons and genes, in this gene-centered view of evolution.
Defining a Cistron as a segment of DNA coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria and prokaryotes).
is responsible for a change in recessive trait in a diploid organism (where chromosomes come in pairs).
on the paired chromosome exhibits the recessive trait even though the organism is not homozygous for either mutation.
When instead the wild type trait is expressed, the positions are said to belong to distinct cistrons / genes.
For example, an operon is a stretch of DNA that is transcribed to create a contiguous segment of RNA, but contains more than one cistron / gene.