TRANSLOCATION I OK DROSOPHILA. 



441 



If crossing over takes place at a stage in synapsis in which the 

 chromosomal threads are twisted about each other, we can 

 readily see why the presence of the attached fragment in either 

 one or both of the third chromosomes should produce the effects 

 that it does. Either by stiffening the threads so that they could 

 not twist as tightly as in the rest of the chromosome, or by an 

 actual interference with the loops, the transposed piece should 

 tend to prevent the close association between the homologous 

 chromosomes necessary for that interchange of genes which 

 constitutes crossing over. This interference would produce its 

 maximum effect at the point of attachment, and the effect would 

 decrease to either side. This gradual decrease in the interference 

 is well shown to the left of ebony. The locus of translocation is 

 too close to the right-hand end of the chromosome to show any 

 marked diminution of its effect toward that side. The rough- 

 claret crossovers are, however, nearer to the normal percentage 

 than those in the ebony-rough interval. 



In the cross shown in Table V., 36 crossovers between ebony 

 and rough were observed in which it was possible to follow the 

 behavior of the translocation. The results are shown below. 



TABLE VI. 



Cy ru h st p ss Tr 



Cy A H e Tr 



36 



Translocation is, according to these figures, located at approxi- 

 mately 11/36 of the distance between ebony and rough, or at 

 76.9 on the chromosome map. The data given in Bridges and 

 Morgan's monograph on the III. chromosome characters also 

 show translocation to be between ebony and rough, but their 

 figures show the locus of translocation to be closer to rough than 

 to ebony. Also, the amount of crossing over in the ebony- 



