Chapter *38 



REGULATION OF GENE 

 ACTION-POSITION EFFECT 

 IN DROSOPHILA 



S 



|ome of the chromosomal rear- 

 rangements induced in Dro- 

 sophila by ionizing radiations 

 have the same, or nearly the same, points 

 of breakage, and many nearly-identical rear- 

 rangements are associated with the occur- 

 rence of the same phenotypic change. 

 Moreover, the new phenotype is transmitted 

 whenever the rearrangement is, and is often 

 similar to that produced by a known allele 

 located at or near one of the breakage points. 

 In such cases, the change in phenotype seems 

 to be directly connected with the mutation 

 of a gene known to be located at or near 

 a point of breakage. 



We cannot claim that chromosome break- 

 age in or adjacent to a gene automatically 

 changes it to a particular allele because 

 other breaks at this locus partake in other 

 types of rearrangements which do not pro- 

 duce such a phenotypic change. For the 

 same reason, it is untenable to presume that 

 the radiation which caused the break, simul- 

 taneously produced a minute deficiency or 

 duplication of the affected locus. An im- 

 portant feature of this phenotypic change, 

 therefore, is its disassociation with breakage. 

 However, the change may have something 

 to do with the broken ends that join — oc- 

 curring only when the broken end carrying 

 a given locus unites with the broken ends 

 473 



from certain specific loci in the genome. 

 If we accept this view, then we would expect 

 the gene at the broken end to produce one 

 phenotypic effect when united with certain 

 broken ends and another phenotypic effect 

 when joined to others. In other words, the 

 phenotypic effect of a gene may be modified 

 when it has new linear neighbors. Such a 

 phenotypic change is called position effect 

 (pp. 384-385). Presumably position effect 

 changes the working of a gene without 

 changing the gene itself. Position effect, 

 therefore, may be one of the phenotypic 

 consequences of mutations involving struc- 

 tural rearrangements, even though not a mu- 

 tation itself. 



A gene showing position effect is pre- 

 sumed to be chemically and physically un- 

 changed in any permanent way. Since genes 

 located some distance from a point of break- 

 age sometimes show position effects, position 

 effect can spread somehow along the chro- 

 mosome and affect the functioning of a gene 

 whose immediate linear neighbors have not 

 been switched. This spreading effect is fur- 

 ther reason for dismissing explanations of 

 position effect based solely upon breakage 

 or upon other mutational changes connected 

 with ionization. 



If the physico-chemical nature of a gene 

 showing position effect is unchanged, two 

 predictions can be made. First, the gene 

 in a position-effect rearrangement should re- 

 sume its original function upon being placed 

 near its former genie neighbors in the chro- 

 mosome. This can be tested experimentally 

 either by irradiating individuals carrying re- 

 arranged chromosomes and examining the 

 progeny for structural changes that reverse 

 this rearrangement, or by moving (by cross- 

 ing over) the gene showing position effect 

 to a normal chromosome. In both cases 

 it is found that the gene returns to its orig- 

 inal position and phenotypic effect. The 

 second prediction is that a normal gene 

 placed in the rearranged position by means 



