82 GENETIC VARIATIONS 



are separate for a short distance. It is not difficult to produce 

 individuals in which one of the two component chromosomes 

 is normal (without the mutation), while the other component 

 has the mutation and hence the changed arrangement of the 

 bands. These present a striking appearance. 



Thus in these cases the mutation, long known, is shown 

 to be the result of a break in the chromosome; a break whose 

 occurrence is demonstrated by the changed order of the 

 bands. (Of course the break and rearrangement have 

 occurred when the chromosomes were in the extended condi- 

 tion indicated in Figures ii and 12.) 



In sum, it is clear that mutations are in many cases the 

 result of breakage of the chromosomes, with rearrangement 

 of some of the genes. In cases in which the first knowledge 

 is of the occurrence of breaks and rearrangements, these are 

 found to be accompanied by "gene mutations." And in cases 

 in which the first knowledge is that a mutation has occurred, 

 these are found to be accompanied by breaks and rearrange- 

 ments. Different mutations of the same gene are found to 

 result from the fact that the breakage occurs in different 

 regions of the chromosome, in different cases. The relation 

 of the so-called mutations to breaks and injuries is fully 

 demonstrated. 



Certain further important questions at once arise. Are the 

 so-called gene mutations all of this type? Or are there two 

 classes, one occurring under the action of radiation and 

 resulting from breakage or injury to the chromosomes; the 

 other of a different type ? Evidence on this will come soon, 

 from examination of the conditions in the salivary chromo- 

 somes, in the many stocks bearing diverse gene mutations. In 

 the meantime, the fact that similar and identical mutations 

 occur both under radiation and spontaneously, speaks strongly 

 against the existence of two classes differing in nature. 



