CHAPTER XVIII 



FRAGMENTATION AND TRANSLOCATION 



In the foregoing chapters we have described the chromosomes and 

 their normal behavior through the life cycles of representative organisms, 

 together with the application of such nuclear phenomena to the problems 

 of Mendelian heredity. Several chapters will now be devoted to less 

 common types of chromosome behavior and their genetic consequences. 

 Such phenomena afford valuable natural checks upon current theories. 

 Moreover, it has been found that the frequency of their occurrence can in 

 some instances be markedly increased by artificial 

 means, enabling the investigator to acquire an abun- 

 ^V> /L^ dance of material for cytogenetic studies. They also 

 a\ V^^ constitute a basis for alterations of character beyond 

 ^^ *^^D those attributable to gene mutation and Mendelian 

 •.W^W^^ recombination.^ 



^^ I \ This is a relatively new field in cytogenetics, and 



data as well as interpretations are multiplying very 

 rapidly. In the following pages will be given examples 





• • 



DnCo A 



Fig. 177. Fig. 178. 



Fig. 177. — Translocation induced by X-rays in Crepis tectorum. 1, normal somatic 

 complement. 2, a portion of a C-chromosome has been translocated to a Z>-chromosome. 

 {After M. Nawaschin, 19.31c.) 



Fig. 178. — Chromosome fragments in Tradescantia. A, bivalent fragment associated 

 with normal pair in meiosis. B, anaphase of mitosis in microspore, with some of the frag- 

 ments failing to divide. {After Darlington, 19296.) 



of the better known types of aberrant chromosome behavior, the object 

 being to furnish little more than an introduction to the subject. It 

 should be remembered that opinions in such a field are subject to frequent 

 change, and that further refinements in method will doubtless make 

 necessary a redefinition of the categories into which the observed 

 phenomena now appear to fall. 



1 For references to the literature on the effects of various agencies on chromosome 

 behavior and gene activity, see footnotes in Chap. XIII and footnote 20 on p. 311. 



314 



