186 



CHAPTER 22 



one type of functioning, whereas, when it 

 joins to other genes, it does so in another 

 way, leading to a different type of function- 

 ing. This kind of explanation would be a 

 mutational one, but is made extremely un- 

 likely by the fact that genes located some dis- 

 tance from a point of breakage sometimes 

 show position effects. You will agree that it 

 is unreasonable to believe that a change in 

 the kind of connection between genes at the 

 point of breakage would somehow spread 

 along the chromosome to affect the configu- 

 ration of a gene whose adjacent genes had not 

 been substituted by others. (This spreading 

 effect is additional reason for dismissing ex- 

 planations of position effect based upon 

 breakage itself or upon other mutational 

 changes connected with tracks of ioniza- 

 tion.) 



If the physico-chemical nature of a gene 

 showing position effect is unchanged, then 

 two other predictions should prove true. 

 The gene in a position-effect rearrangement 

 should return to its old functioning upon 

 being placed near its old genie neighbors. 

 This can be done in two ways. Rearrange- 

 ment-carrying individuals can be irradiated 

 and progeny examined for structural changes 

 that reverse this rearrangement. Or the 

 gene showing position effect can be moved to 

 a normal chromosome by means of crossing 

 over. In both cases it is found that the gene, 

 returned to its old position, returns to its old 

 way of functioning. A second prediction is 

 that when a normal gene is placed in the re- 

 arranged position via crossing over, it should 

 then cause the position effect. It does. 



In Drosophila, the organism most studied 

 in this respect, position effects often accom- 

 pany rearrangements that bring genes in 

 euchromatin near those in heterochromatin. 

 Placing a gene normally located in a euchro- 

 matic region near or in a heterochromatic 

 one often produces a special, wavering, posi- 

 tion effect which is expressed in the pheno- 

 type in a mosaic or variegated way. Thus, 



for example, if the gene for dull-red eye color 

 on the X chromosome, w+, normally located 

 in euchromatin, is placed in the heterochro- 

 matin proximal to the centromere by means 

 of a paracentric inversion, the eye color 

 produced is mottled, being composed of 

 speckles of white and dull red. Such variega- 

 tion is reduced, however, if, by breeding, an 

 extra Y chromosome or another heterochro- 

 matin-rich chromosome is added to the geno- 

 type. It is not yet known how this suppres- 

 sion of variegation is produced. 



We have seen that the only requirement for 

 the occurrence of position effect is an appro- 

 priate shift in the kind of linear neighbors a 

 gene has. Breakage merely provides us with 

 a way of obtaining such shifts. The possi- 

 bility should be entertained that position 

 effects could also be detected with the aid of 

 some other mechanism for changing the rela- 

 tive positions of genes. We already know that 

 crossing over can do this. Let us see if we 

 can devise a particular crossover system ^ 

 whose operation might produce a position 

 effect. 



An X-linked mutant is known in Drosoph- 

 ila, which has the effect of reducing the 

 number of facets {ommatidia) in the com- 

 pound eyes. Because it changes the eye 

 from round to a slit shape it is called Bar 

 (B). When the normal and the Bar-con- 

 taining chromosomes are studied in larval 

 salivary gland nuclei, it is found that about 

 six successive bands in the normal chromo- 

 some are duplicated in tandem in the Bar 

 chromosome. Let us designate as 123456 

 each such region of six bands, so that a nor- 

 mal female would contain 123456/123456 

 and a homozygous Bar female 123456 123456 

 /123456 123456. In normal (+/+) females 

 the two homologs would pair with homolo- 

 gous numbers (parts) together, and crossing 

 over could take place between correspond- 

 ing numbers. In homozygous Bar (B/B) 



' Based upon investigations of A. H. Sturtevant, 

 H. J. Muller, C. B. Bridges, and others. 



