顺反子，也做作用子，是基因的一个旧名称，它于1955年由美国分子生物学家本兹尔提出的，他称基因内部的功能互补群为顺反子。顺反子通过顺反试验确定，如两个位点可以互补，则两个位点不属于一个顺反子；如两个位点不可以互补，则两个位点属于一个顺反子。

它是遗传物质的最小单位。一个完整的顺反子是传递遗传信息的前提，即多肽链的氨基酸顺序的正确编排。

顺反子和基因关系与基因的定义有关。

有一种对基因的定义认为，基因与顺反子为同一物，是一个最小的遗传功能单位。

但是另一种定义则认为，顺反子是比基因更小的功能单位，一个基因包含几个遗传的功能单位，即顺反子。

A cistron is 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.

For example, suppose a mutation at a chromosome position is responsible for a 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, , is responsible for the same recessive trait. The positions and are said to be within the same cistron when an organism that has the mutation at on one chromosome and has the mutation at position 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.