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CELL MECHANICS 



double breakage and crossing-over. Rare natural crossing-over 

 would be due to rare natural breakage, which should lead to the 

 same regular result. Where crossing-over follows natural breakage 

 the frequency of inversion, translocation and other structural 

 changes resulting from such breakage should be correspondingly 

 reduced for the reasons we shall next consider. 



(c) Structural Change. Structural changes can be classified from 

 three points of view, according to their modes of origin, spatial 

 relationships, and mechanical and genetical results. These have so 

 far been treated separately. We may now see how they have to do 

 with one another. The chromosomes with which we begin are the 

 normal efficient chromosomes with one centromere and two ends. 

 From these we obtain new types with no centromere or with two, 

 and with no ends or with three or four. According to these 

 mechanical results we can therefore classify chromosomes in the 

 following way, giving the first seven types initial symbols (Table 77). 



Table 77 

 Mechanical Types of Chromosomes 



To consider next the genetic relationships of structural change, 

 some of these are simple and others very complex. Deficiency, the 

 loss of a terminal segment, is the simplest, since it can affect only 

 one chromosome, dividing it into a monocentric and an acentric 

 fragment. Deletion, the loss of an intercalary segment of a chromo- 

 some, requires that the two breaks shall be intra-radial, in the same 

 arm of the chromosome. Inversion, which is always so far as we 



