A method of genetic analysis that determines whether recessive mutations occur in a single gene or in different genes. The test was devised by the American geneticist E. Lewis in 1951.
In conducting the cis-trans test, the mutations under study are combined in the trans and cis positions. When the mutations are combined in the trans position, two individuals, each with one of the mutations under analysis, are crossed. When the mutations are combined in the cis position, an individual with both mutations is crossed with an individual of a wild (normal) type. In the trans test, that is, the functional test for allelism, if the mutations combined in the trans position belong to different genes, a hybrid organism is automatically obtained from an intact copy of each gene. In this case the recessive mutations are not manifested and the hybrid has a normal phenotype, that is, the mutations are complementary. If the combined mutations belong to the same gene, both copies of the given gene in the hybrid are negatively affected and a mutant phenotype appears, that is, the mutations are noncomplementary. hence related to crossing over.

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Apoorva Arora
IIT Roorkee
askIITians Faculty