326 GENETICS 



of sets of chromosomes must play an important role in pro- 

 duction of diverse varieties, in both wild and domesticated 

 animals and plants. 



2. Breakage and Reunion of Parts of the Chromosomes. 

 Interchange of Blocks of Genes among the Different 

 Chromosomes. Deficiencies, Inversions, Transloca- 

 tions; Their Effects on Characteristics 



Another set of changes in the genetic system involves 

 breakage and rearrangement of the parts of the chromo- 

 somes, at times with loss or destruction of some of the 

 genes, or with modification of the action of certain genes. 

 Such changes can be produced by subjection of the develop- 

 ing organism to radiations: to X-rays or the rays from 

 radium. They also occur at times from unknown causes in 

 organisms not subjected to radiations. Their production by 

 radiations has made it possible to investigate on a large 

 scale the results of such changes. A large part of the work 

 on such matters has been done on Drosophila, because the 

 genetic system and its relation to characteristics is better 

 known in this organism than in any other.* 



Subjection of organisms to X-rays or to rays from ra- 

 dium, if used in such a strength as to produce changes in 

 the genetic system, is very injurious to them. The radiations 

 kill many of the organisms. They leave many of the rest 

 sterile. Other survivors are found to have suffered injuries 

 of various kinds. Among these are injuries to the genetic 

 system. The radiations cause some of the chromosomes to 

 break, cause pieces of different chromosomes to stick to- 

 gether; they destroy some of the genes; they produce in 

 the genetic system irregularities of many kinds. These 

 changes affect the characteristics of the organism and the 

 way in which the characteristics are inherited. These ef- 

 fects are of great interest. To understand them it is nee- 



