RESULTS OF IRREGULARITIES 39 



In other cases chromosomes of the different pairs are found 

 to have exchanged parts ; they have exchanged blocks of genes 

 (Figure 9, A to D). Furthermore, in ways that w^e shall 

 examine later, the order of the genes in a part of a chromo- 

 some sometimes becomes reversed, while in the rest of the 

 chromosome the order remains unchanged (Figure 9, E and 



B 



E 



abcjclefglh tjk Im 



e d li L J k I m 

 -« — • — • — • — • — • — • — • 



t 



Figure 9. Diagrams of the exchange of genes between chromosomes 

 of different pairs (^ to D), and of changes in the order of the genes in 

 a part of the chromosome (E and F). In E and F the order of the genes 

 is given by the letters; in F the order of the genes between c and h is 

 reversed as compared with E. 



F). In this way genes that are at a distance from each other in 

 some individuals are brought close together in other indi- 

 viduals. These changes in the relation of the genes, and the 

 accompanying disturbances, result in striking changes in the 

 method of inheritance, and also, as appears probable from 

 recent discoveries, in the characteristics that are inherited. 

 The genetic variations so caused appear to play a very great 

 role; some of the so-called gene mutations are apparently of 



