A cistron is an alternative term to a gene. The term cistron is used to emphasize that genes exhibit a specific behavior in a cis-trans test; distinct positions (or loci) within a genome are cistronic.
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History
The words cistron and gene were coined before the advancing state of biology made it clear that the concepts they refer to are practically equivalent. The same historical naming practices are responsible for many of the synonyms in the life sciences.
The term cistron was coined by Seymour Benzer in an article entitled The elementary units of heredity. The cistron was defined by an operational test applicable to most organisms that is sometimes referred to as a cis-trans test, but more often as a complementation test.
Definition
For example, suppose a mutation at a chromosome position
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. The operon is said to be polycistronic, whereas ordinary genes are said to be monocistronic.