GENETIC VARIATIONS 335 



duced on genes near the points of breakage the following 

 effects : 



1. A dominant eye-color change or mutation. 



2. Instability of four different genes. 



3. A change that was lethal when homozygous. 



No other case has yet been so thoroughly studied as the 

 one just described. But in a considerable number of cases 

 breakage of a chromosome is known to make certain genes 

 near the points of breakage unstable, so that they operate 

 in certain cells, but not in others. Thus they produce a body 

 that has diverse inherited characteristics in its different 

 parts, a condition commonly spoken of as mottling. 



In other cases breakage of a chromosome causes a simple 

 change in some inherited characteristic, giving a dominant 

 or a recessive "mutation." For discussion of these, see the 

 next chapter. 



What is the nature of the effect on the genes that Is pro- 

 duced by breakage of the chromosome? On the one hand, 

 chromosome breakage is in the nature of a laceration, an 

 Injury, to the tissues. The effects on the genes near the point 

 of breakage may be essentially small scale injuries, affecting 

 single genes, induced at the same time as the greater In- 

 juries seen in breaks, deletions and deficiencies. The fact 

 that some of the genes are put completely out of action, so 

 that they will no longer permit development, and that 

 others are made weak and unstable, seems to suggest such 

 an interpretation. 



Another possibility that Is at present under examination 

 is the following. When a chromosome breaks, the pieces 

 usually reunite in an unusual position, so that genes near the 

 region of the break are in close relation to other genes than 

 those that are normally near to them. It Is suggested that 

 possibly the effect of a gene depends on its position with 



