STEPHENS: RESUME OF THE SYMPOSIUM 501 



function is impaired. In the diploid condition at least one of the 

 homologous cistrons must preserve the normal sequence of base 

 pairs if the normal phenotype is to be produced. If the sequences 

 in both cistrons are interrupted, either at identical or different 

 sites, a mutant phenotype is expressed (the trans effect). If the site 

 of interruption is different in the two cistrons, then breakage between 

 the sites followed by recombination may restore the normal sequence 

 in one cistron and produce a double interruption in the other. 



The joint action of two homologous cistrons, one with a normal 

 sequence and the other with a double interruption, would produce 

 a normal phenotype (the cis effect). According to this concept, the 

 whole lozenge "complex" in Drosophila would be considered a single 

 cistron and its "loci" merely sensitive sites or "hot spots". From 

 the phenotypic point of view, mutant combinations exhibiting 

 different cis and trans effects should result from rearrangements 

 within a pair of homologous cistrons. On the other hand, if the 

 "mutant" effects are restricted to recombinations in the serial order 

 of adjacent cistrons or cistron "repeats", there should be no pheno- 

 typic distinction between cis and trans arrangements. On this basis, 

 the alpha and beta "elements" in the A complex of corn would 

 be considered repeated cistrons — not because they are separable but 

 because cis and trails combinations of their "mutants" appear to be 

 identical phenotypically. 



Although the application of the cistron concept to higher organ- 

 isms leads to certain interpretative difficulties, e.g., in the case of 

 dominant mutants, it might provide a unifying principle leading 

 to a better understanding of position effects. Breakage within a 

 cistron followed by separation of the broken segments through 

 translocation should produce mutant effects; breakage between intact 

 cistrons followed by translocation should not. It might be easier 

 chemically for broken cistron ends to be linked to a heterochro- 

 matic region than to a dichromatic region. If so, broken cistrons 

 with accompanying mutant effects might be recovered selectively 

 in heterochromatic regions, hence accounting for the association 

 between position effects and heterochromatin. Further, the fre- 

 quency of position effects in Drosophila and their comparative rar- 

 ity in other organisms might have nothing to do with relative 

 heterochromatic properties and be only partly explained by the 



