XXXVIII 



INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 



Theodosius Dobzhansky 

 California Institute of Technology, Pasadena 



Introduction. Classification of chromosomal aberrations. Influence of X-rays on 

 disjunction of chromosomes. Methods of detection and of studying chromosomal rear- 

 rangements. The mode of origin of chromosomal rearrangements. Permanence of the 

 spindle fibers. Cytological maps. Cytological demonstration of crossing over. Gene 

 changes associated with chromosome breakages. Mechanism of meiosis. References. 



INTRODUCTION! 



The process of evolution is the resultant of several intimately inter- 

 woven, but virtually independent, processes. Since evolution implies 

 the presence of hereditary differences between the ancestral and the 

 derived forms, and since the only known method of the origin of heredi- 

 tary differences is gene mutation, the process of mutation must be 

 considered almost by definition the mainspring of evolutionary changes. 

 It would be, however, fallacious to argue that the concepts of evolution 

 and of gene mutation are synonymous. For we know at the present at 

 least one more kind of change the significance of which seems to grow 

 with the progress of exact studies on the mechanism of evolution. Com- 

 parative genetic as w^ell as comparative cytological investigations on 

 related forms of animal and plant life show that besides genie diversities, 

 differences in the arrangement of chromosomal materials are encountered. 

 No account of the causes of evolution can be either complete or satisfac- 

 tory unless these latter differences are taken into consideration. 



Rearrangements of the chromosomal materials offer, moreover, a 

 profitable field for studies on the mechanism of heredity. The so-called 

 chromosomal abnormalities are easily analyzable by a combination of 

 genetic and cytological methods, and the differences between the 

 "normal" and the "aberrant" structures can be described in sufficiently 

 exact terms. The functioning of these altered structures can then be 

 studied. Experience has demon.strated that the insight into the mecha- 

 nism of heredity gained by this method is more profound than that gained 

 by observations on the normal chromosomes only. This fact alone 



' The manuscript of this article was first prepared and submitted to the editor 

 in August, 1933. In the revision, November, 1934, mention of some of the new 

 literature was made, but no adequate discussion given. It has been impracticable 

 to specify this new material, but for the most part the dates of publication will ade- 

 quately indicate the newer work receiving such brief attention. 



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