Cistron

The words cistron and gene were coined before the advancing state of biology made it clear to many people, though now disputed, 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.[2] 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. Richard Dawkins in his influential book The Selfish Gene argues against the cistron being the unit of heredity (he also argues against group selection), and it being the best definition of a gene. I.e. he doesn't argue against the existence of cistrons, or them being elementary, just that natural selection selects them (except back in an era when life was much more primitive). He defines a gene as a larger unit (he also defines replicators, more general than cistrons and genes), that other may now call gene clusters, as the unit of heredity.

For example, suppose a mutation at a chromosome position ${\displaystyle x}$ is responsible for a change in recessive trait in a diploid organism (where chromosomes come in pairs). We say that the mutation is recessive because the organism will exhibit the wild type phenotype (ordinary trait) unless both chromosomes of a pair have the mutation (homozygous mutation). Similarly, suppose a mutation at another position, ${\displaystyle y}$, is responsible for the same recessive trait. The positions ${\displaystyle x}$ and ${\displaystyle y}$ are said to be within the same cistron when an organism that has the mutation at ${\displaystyle x}$ on one chromosome and has the mutation at position ${\displaystyle y}$ 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. Or simply put, mutations on the same cistrons will not complement; as opposed to mutations on different cistrons may complement (see Benzer's T4 bacteriophage experiments T4 rII system).

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.