138 Nature of the Genetic Material 



position effects, there is one difference that could be responsible for 

 its specific features. Tlie ordinary position effect involves a break 

 near a given locus, and so does the Dubinin effect. The variegated 

 variant of the ordinary position effect depends upon a break in 

 heterochromatin near v^'hich a euchromatic locus comes to life. In 

 the ci case the ci locus is adjacent to chromocentral heterochromatin. 

 The break occurs in the heterochromatin between the locus and the 

 spindle fiber so that after translocation to another chromosome the 

 order is now: some euchromatic section — segment of fourth chromo- 

 some heterochromatin — ci locus (the latter two in their normal or- 

 der). In contrast to the other position effects, the locus of ci remains 

 flanked by its normal chromosomal structure on both sides, but on 

 the spindle attachment side the heterochromatin is broken and at- 

 tached to euchromatin of another chromosome. Expressing the situ- 

 ation in terms of action of the break (solely for clarity's sake and 

 using terminology which we consider to be wrong), we should state 

 that in the ordinary position effect the break acts directly upon the 

 locus, in the variegated effect also, but with heterochromatin at 

 its back, and in the Dubinin effect through a layer of heterochromatin. 

 Therefore, we might expect phenotypic results of a peculiar type 

 for the ci effect. 



This is indeed the case, which follows from the elaborate work 

 of Stern and collaborators (Stern, 1943, 1948; Stern et al, 1943-1946). 

 In this work the primary interest is in genie action, a problem which 

 is attacked by dosage experiments (1, 2, 3 doses of ci, +, and R(ci) 

 [rearrangement near ci] in different combinations). We are not con- 

 cerned here with the theory explaining the different dosage results, 

 but orily with the comparison of the actions of the mutant locus 

 versus position effect by rearrangement. Before comparing these, 

 some other points must be reaUzed first. The ci mutant is very 

 easily affected by temperature. At low temperatures it is slightly 

 dominant, so that only actions at stated temperatures can be com- 

 pared. This fact indicates to the physiological geneticist special thresh- 

 old conditions for the ci action. In agreement with this a number of 

 isoalleles were found, multiple alleles which act at normal tempera- 

 tures above the threshold in heterozygous condition, but show their 

 different actions — some always above the threshold, some nearer to 

 the threshold value — in temperature experiments and different com- 

 pounds. One more fact should be mentioned (to be discussed and 

 explained in the chapter on dosage). A mutant opposite a deficiency 

 usually has a more extreme phenotype than the homozygous mutant. 



