340 CELL HEREDITY 



Clearly, when the Bar regions are together on one chromosome, there 

 is a greater effect, i.e., a greater reduction in number of facets, than 

 when each is on a separate chromosome. Sturtc\ant called this phe- 

 nomenon position effect. 



L'ntil recently, the manv examples of position effect could be grouped 

 into two categories, stable or S type, and variegated or \' type. 



Studv of the S-type effects led to the discovery of position pseudo- 

 alleles, which have been discussed in Chapter 6. They reflect the large 

 size of the functional gene, in contrast to the nucleotide dimensions of 

 the units of mutation and recombination. Continuity of unaltered DNA 

 along one chromosome appears to be necessary for the formation of a 

 functional gene product. The cis-trans test of allelism demonstrates 

 whether or not the mutated regions of two alleles are within the same 

 functional region. 



The V'-type position effect was first found by Muller in 1927, in 

 X-rayed Drosophila. When X-ray studies of Drosophila became popular 

 in the 1930's, variegated position effects were widely observed and in- 

 vestigated bv a number of workers. In general, this phenomenon in- 

 volves chromosome rearrangement, with the insertion of the marker 

 under observation into a new location, usually into a region considered 

 to be heterochromatic (see p. 95). The change usually seen is the par- 

 tial repression of the wild-type phenotype, as if something were inter- 

 fering with the normal expression of the gene. The result is often a 

 mottled appearance, most easily recognized in effects on pigmentation. 



The example found by Muller was later shown to result from a trans- 

 location of the white locus (eye color). A remarkable feature of the 

 phenomenon is that the heterochromatic region may exert a spreading 

 effect along the chromosome, interfering with the expression of a number 

 of linked loci, to extents which decrease with distance. The potency 

 of the heterochromatin varies with its location, but the significance of 

 this fact is not known. 



One of the most spectacular examples of the spreading effect is shown 

 in Figure 11.18, in which the region from band 3Cj to GAj of the 

 X chromosome was inserted into a heterochromatic region of the fourth 

 chromosome. Mottling was found for genes at both ends of the inserted 

 piece, but not for the group which are in the center. Since the genes 

 involved influence a wide variety of phenotypes, the response seems to 

 be determined entirely by location. 



That variegated position effects are reversible was first shown by 

 Dubinin in Russia, and later by others. The method consisted essen- 

 tially of selecting recombinants after crossing over, in which the trans- 

 located gene had been moved out of the heterochromatic regions. 



