FRAGMENTATION AND TRANSLOCATION 



323 



derived (Muller, 1930c). This indicates that in such cases the disjunctive 

 force may not be limited to the spindle-attachment region of the chromo- 

 some, homologous distal portions tending to pass toward opposite poles 

 even when one of these portions has been translocated (Blakeslee, Darling- 

 ton, Muller). Many more observations must be made before safe 

 generalizations can be made regarding these matters. 



Bearing on the Chromosome Map. — In some of the foregoing examples 

 of translocation the abnormalities were detected first in altered genetic 

 behavior and then confirmed cytologically, while in others they were first 

 seen cytologically and then confirmed genetically. This is a striking 

 corroboration of the cytogenetic theory that certain genes are definitely 



Nl 



■100.7 (ca) 

 -9I.I(ro) 



-70.7(6")^ ^ 

 (ru) 



-26.5- 

 (h) 



-44 



(CU) 



sr 



70.7- 



,91.1-1 



100.7- 

 (ctt) 



K IT 



■# 



N^ 



B 



-non-coi 



I nr„ m iz 32 



ro- 



-h- 



-sf- 

 :P = 



CU 



•sr- 



Fig. 184. — Chromosome aberrations induced by X-rays in Drosophila. A, a "III 

 to II" translocation. In the drawing of the chromosome complement at the right the 

 translocated piece (indicated by arrow) came from the V-shaped chromosome at upper 

 right. B, hyperdiploidy produced by crossing a fly having the translocation with a normal 

 fly. {After Painter and Muller, 1929.) 



located in certain chromosomes. In each case where the data were 

 adequate the genetic results of translocation and inversion showed that 

 the genes must have been arranged in the oi'der previously assigned to 

 them in the "genetic map" of the chromosome on the basis of crossover 

 values. On the other hand, they showed that these genes did not 

 necessarily have the spacing shown in the map. For example, the 

 translocated piece in Fig. 184, A was actually smaller than would be 

 expected from the large portion of the genetic map altered. This and 

 other cases in Drosophila have shown that the genes in the middle portion 

 of the "genetic map" of chromosomes II and III are represented closer 

 together than they are in the "cytological map" of the actual metaphase 

 chromosome (Fig. 185). The reason for this seems to lie in the fact 

 that in these chromosomes crossing-over is less frequent in the middle 

 region near the spindle-attachment point than it is toward the ends. 

 The genetic map was originally built up on the hypothesis that crossing- 



