FRAGMENTATION AND TRANSLOCATION 321 



inversion is responsible for a change to the atelomitic from the telomitic 

 position, which is regarded as primitive for Acrididae. 



Translocation. — The transfer of a piece of one chromosome to another, 

 aside from that involved in normal crossing-over, is called translocation 

 (Fig. 179, 5). It seems probable that as a rule the transfer is accom- 

 plished at one step and does not represent the attachment of a fragment 

 previously free. Commonly the translocated piece is borne terminally by 

 the receiving member; rarely it extends laterally from some intermediate 

 point (Fig. 67, D). The transfer may be to either a homologous or a non- 

 homologous member of the complement. It has been known to occur 

 naturally in many instances, but its frequency can be markedly increased 

 by the use of X-rays and heat. The first known case was discovered 

 genetically in non-irradiated Drosophila (Bridges, 1919, 1923). The 

 transfer of a single piece, or "simple translocation," is to be distinguished 

 from the mutual exchange of pieces known as "reciprocal translocation" 

 (Fig. 179, 6). The latter type will be dealt with in the following chapter. 



Apparently any portion of one chromosome may be translocated to 

 any other chromosome. Because of the role of the spindle-attachment 

 region, many translocations must result in mitotic derangements serious 

 enough to prevent development. Hence in translocations induced by 

 X-rays it is commonly the "distal" portion without the attachment region 

 which appears to have been transferred, the "proximal" portion with the 

 attachment region remaining as an altered independent chromosome.^ 



Among the most favorable plant species for the study of transloca- 

 tions are Crepis capillaris and C. tedorum since they normally have 

 respectively but three and four pairs of easily distinguishable chromo- 

 somes. The results of many modifications are thus readily detected in 

 the metaphase^ (Figs. 69, 198). For the investigation of alterations 

 involving only very minute portions of the chromosomes, reliance must be 

 placed on such forms as Zea Mays with larger chromosomes exhibiting 

 individually their characteristic aspects with great clearness in the meiotic 

 prophase. 



Among simple translocations involving only homologous chromo- 

 somes the first to be worked out both cytologically and genetically was 

 that in which a portion of the F-chromosome was found to be attached 

 to the X-chromosome in Drosophila (Stern, 19266, 1927a). Since 

 that time a number of other examples involving various pairs have 

 been discovered. The translocations can often be detected in the altered 

 length of the chromosomes concerned and, when their morphology is 

 sufficiently well known, in their altered form as well (Fig. 183). 



* E.g., Dobzhansky (1931a) on Drosophila, M. Nawaschin (1931c) and Lewitsky 

 and Araratian (1931) on Crepis. 



" M. Nawaschin (1926, 1930, 1931ac), Lewitsky and Araratian (1931). 



