CROSSING-OVER 



557 



In translocation a segment of one chromosome is never attached to 

 the end of another. He has concluded that true ends cannot rejoin 

 and therefore have a specific non-fusability. Further, the indirect 

 proportion of induced structural changes to X-ray dosage suggests 

 that they sometimes depend on two hits, not, like gene-changes, on 

 one. This at least is true of the changes that survive in breeding 

 experiments. Simple deficiencies from one breakage would not 

 usually survive. 



Table 78 



Analysis of certain Simple Structural Changes in Terms 

 of Breakage and Reunion {cf. Table 77). 



Catcheside (1935, 1936), on the other hand, explains the reciprocity 

 of structural changes without assuming a specific non-fusabiUty of 

 the ends. He infers that the reciprocity of changes is simply due to 

 the high unlikelihood of an end of one chromosome lying near the 

 point of breakage of another. Stadler considers (from the behaviour 

 of reverse mutation islands) that there is an unlimited time-interval 

 during which breakage-ends could reunite. This view seems unten- 

 able in his particular examples, since acentric fragments are nearly 

 always lost at mitosis. 



Ring chromosomes in Zea, perhaps through interlocking at 

 anaphase, form double dicentric rings which break Uke the bridges 



