50 MOLECULES, VmUSES, AND BACTERL^ 



A. One mutated site in each of two loci 



locus 1 locus 2 

 — M \ M — 



CIS 



^ \ K 



trans 



^^ 



B. Two mutated sites in one locus 



locus 1 locus 2 



M H 1 \- 



cis 



-M^ 



trans 

 -H \ \- 



Figure 4. Diagram of the cis-trans test for allelism. In A, the phenotype will 

 be non-mutant in both cis and trans arrangements. In B, the cis arrangement 

 alone gives a non-mutant phenotype. 



brid, for in either the cis or the trans position each locus is present 

 in normal state in one or the other of the chromosome pair. How- 

 ever, if the two mutated sites are in the same functional gene, the 

 phenotype of the hybrid will be normal only if the mutated sites 

 are in the cis position, for it is only in this position that the hybrid will 

 contain one normal gene. Thus when two mutations involve the same 

 gene locus, the phenotype of the hybrid will depend upon the relative 

 positions of the mutational sites, and a cis-trans eflFect will be observed. 

 The cis-trans test has proved very useful in that it has compelled 

 us to take into account the difference between the functional gene and 

 the mutational sites. It has exerted a powerful influence in crystallizing 

 our ideas concerning the various sorts of units of which the chromo- 

 some is composed. In practice it proves not to be a clear-cut definition 

 of functional units of the chromosomes, for fairly often two mutants 



