GENETIC VARIATIONS 327 



essary to consider the method of operation of the radiations. 



The number of breaks and injuries produced in the chro- 

 mosomes is proportional to the energy of the radiation; 

 that is, to the intensity of the radiation multlphed by the 

 time during which the cells are exposed to it. It was at first 

 supposed that the effects on the chromosomes and genes 

 were caused by direct "hits" on them by the free electrons 

 resulting from the rays. But fuller study of the results shows 

 that this cannot be the case, as appears from the following 

 facts. 



While under radiation the chromosomes are slender 

 threads bearing the genes at intervals (figure 37). In well 

 studied organisms, such as the fruit-fly, the order in which 

 the different genes occur on the thread is well known; it is 

 represented in such maps as those of figure 38. Subjection 

 to the radiations causes a number of changes of remarkable 

 and unexpected character, as follows: 



( 1 ) The radiation may cause a break in a chromosome 

 in certain of the cells. It is an extraordinary fact that when 

 this occurs in a given cell, there is usually or always in the 

 same cell another chromosome break, at some distance from 

 the one first mentioned. This other break may occur at an- 

 other point in the same long thread-like chromosome. Or it 

 may occur in another chromosome within the same cell. 



Absolutely considered, chromosome breaks occur in very 

 few of the cells. The fact that there are usually two breaks 

 in the same cell thus cannot be due to chance. Whatever 

 causes one break must cause another. It can be shown that 

 the chance of a single electron so striking the chromosome 

 as to cause two breaks some distance apart is so minute that 

 it can be excluded as the cause of the frequent occurrence of 

 two breaks. The association of one break with another in 

 the same cell requires explanation. 



(2) Furthermore, after the two breaks have occurred, 

 the pieces separated by the two breaks usually unite again 



