A test used in genetics to determine whether two independent mutations, affecting the same phenotypic function, occur within the same gene or in different genes (see cistron). A pair of homologous chromosomes are brought together in the same cell (normally a bacterial cell). Mutations located on the same chromosome are described as being in the cis position; if they occur one on each homologue they are in the trans position. If the cell is phenotypically normal for both cis and transpositions of the mutations, these mutations are considered to be present in different cistrons. If the resultant cell is normal when the mutations are in the cis position but mutant when the mutations are in the trans position, the two mutations are considered to lie in the same cistron. See illustration.

Cis-trans test