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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.
Lewis’ trans test may be improved by studying mutations in the process of combining in the cis position as well, thus eliminating artifacts (atypical structures) caused by gene interaction at the level of gene products. If in the cis-trans test the hybrid’s phenotype is the same in the cis and trans positions, that is, if there is no cis-trans effect, the mutations under study occur within different genes. If, however, the hybrid’s phenotype is different in the cis and trans positions, that is, if there is a cis-trans effect, the mutations occur within the same gene. In 1957 the American geneticist S. Benzer proposed that the unit determined by the cis-trans test be called the cistron.
Since it is difficult to combine in the cis position mutations that are situated closely together within a chromosome, the cis-trans test is rarely conducted. Research has revealed that in some cases several mutations of the same gene are complementary, and also that the cis-trans effect may occur in mutations of different genes in a single operon. These discoveries have reduced the theoretical value of both the trans test and the cis-trans test. The trans test is still widely used in applied genetic analysis to determine whether given mutations occur within a single gene (cistron).
REFERENCESStent, G. Molekuliarnaia genetika. Moscow, 1974. (Translated from English.)
Fiziologicheskaia genetika. Leningrad, 1976.
T. R. SOIDLA