Regulation of Gene Action — Position Effect in Drosophila 

 1 1.5 3.0 



479 



figure 38-5. Crossing over between apricot 

 and white /'// attached-X chromosomes. 



+ + 



H h 



apr + 



apr w 



> 



spl 



trans to the cis form. To do this, we de- 

 tach — that is, separate the arms of — the at- 

 tached-X's in the dull-red-eyed exceptional 

 flies (by collecting the products of the oc- 

 casional crossing over that occurs between 

 the attached-X and the Y in the hetero- 

 chromatic regions near their centromeres) 

 and determining the genes carried in each de- 

 tached arm. The finding that one arm can 

 always be represented as y apr + w+ spl + 

 and the other as y+ apr w spl offers strong 

 support for the view that the dull-red ex- 

 ceptional females were cis heterozygotes, 

 and that apr lies to the left of w on the X 

 chromosome, as shown in the figure. (It is 

 instructive to work out the arrangement of 

 the markers after crossing over between apr 

 and w on the assumption that apr is to the 

 right of w.) 



Proof that the exceptional dull-red fe- 

 males result from position effect rather than 

 mutation is obtained by mating these ex- 

 ceptional females and occasionally obtain- 

 ing daughters with pale apricot eye color. 

 In these new exceptional daughters the orig- 

 inal gene arrangement is found restored by 

 crossing over. 



The phenotypic difference between pale 

 apricot and dull red is undoubtedly the re- 

 sult of position effect, since the only differ- 

 ence between the cis and trans conditions 

 is in the arrangement of the genetic mate- 



rial. This phenomenon is therefore termed 

 a cis-trans position effect. To detect such 

 an effect it is necessary to separate two very 

 closely linked genes. Prior to the experi- 

 ment the genes used had been considered 

 alleles because of their closeness on the ge- 

 netic map and their similar phenotypic ef- 

 fects, but observing their cis-trans position 

 effect proved they were nonalleles occupy- 

 ing different loci. When the other genes 

 making up the "white multiple allelic se- 

 ries" (Chapter 5) are investigated, some 

 are found to be allelic to w and others to 

 apr. Some, however, are allelic to neither, 

 and appropriate crossing over studies show 

 that the "white region" on the X is a nest 

 of five (perhaps more) separate, linearly ar- 

 ranged genes with similar effects. 



Other regions in the genome are now 

 known where two or more genie alternatives 

 — previously considered allelic — prove to be 

 pseudoallele, that is, prove to be nonallelic 

 when subjected to the cis-trans test. In 

 addition to pseudoallelism in Aspergillus, 

 other microorganisms, and corn, examples 

 of pseudoallelism include cases involving 

 color in cotton; lack of tails in mice; lozenge 

 and vermilion eye colors in Drosophila. 



Another case of pseudoallelism in Dro- 

 sophila 4 involves nonalleles whose func- 

 tions differ somewhat more than apr and w. 

 4 See E. B. Lewis (1963) for complete discussion. 



