GENETIC VARIATIONS 333 



genes that were close together In the chromosome are now 

 far apart; certain others that were far apart are now close 

 together. This results in changes in the linkage ratios of the 

 genes. Furthermore, if such a chromosome with part of its 

 genes reversed in order is present in the same cell with an- 

 other of the same pair in which the genes retain their usual 

 order (figure 68), this prevents any exchange of genes in 

 the parts that are not in the same order, so that there is 

 no crossing-over of characters dependent on these genes. 

 Inversions of parts of the chromosomes are frequently dis- 



ci e f g h i j k I 



-•— I — • • • • I « • • ». 



m 



+ 



h S f e i j k I 



m 



Figure 68. Diagram of two chromosomes at the conjugation stage, 

 one containing an inversion (e to //), so that there is no crossing-over, 

 or exchange of genes in that region. The genes e to // of the upper 

 chromosome cannot conjugate normally with the genes // to e of the 

 lower chromosome, so that no exchange occurs between those parts. 



covered through the failure of certain chromosomes of the 

 same pair to exchange genes. Further study then as a rule 

 shows that an inversion has occurred in one of the chromo- 

 somes. 



Another effect of great interest results from breakage of 

 chromosomes. Usually some of the genes near the points of 

 breakage become altered in their physiological action. They 

 no longer affect the characteristics of the organism in the 

 way they did before the breakage occurred. The changes in 

 gene action are of the kind that have been commonly called 

 gene mutations (see next chapter), so that "chromosome 

 breakage causes gene mutations." The nature of the changes 

 in the action of particular genes resulting from chromosome 

 breakage will best be grasped from certain examples. 



Schultz and Dobzhansky ^ have described in detail the 



