CONSTITUTION OF THE CHROMOSOMES 1 45 



of exchange ratios is found when in Drosophila white is 

 paired with different characters, and individuals bearing the 

 two are mated with normals: 



White — Abnormal Abdomen 1.7 



White — Bifid Wings 5.3 



White — Club Wings 14.3 



White — Vermilion Eye 30.5 



White — Miniature Wings 33.2 



White — Rudimentary Wings 41.2 



White — Bar-Eye 43.6 



The exchange ratio of the character white eye with many 

 other characteristics has been determined, but the above 

 list gives typical results. There is a whole series of diverse 

 exchange ratios, varying from about one per cent to nearly 

 50 per cent — each ratio being approximately constant when 

 the given pair of characteristics is employed. 



Why does there exist such series of diverse ratios as is 

 illustrated above? When there is an exchange, the two parts 

 of the given chromosome that produced the two character- 

 istics become separated from each other; they break apart. 

 If the exchange ratio is low, they break apart only in rare 

 cases; if the exchange ratio is high, they break apart fre- 

 quently. The question then becomes: Why do certain parts 

 of the chromosome break apart frequently, others less fre- 

 quently, in the way shown by the series of exchange ratios? 



In judging of this, there are certain important facts to 

 be considered : 



I. It can be proved experimentally that the exchange 

 of parts occurs at the time of the last two divisions in form- 

 ing germ cells (see figure 17). (This is proved by subject- 

 ing the individuals at different periods in their lives to high 

 temperatures, which change the frequency of exchanges. 

 This result occurs only if the subjection to high tempera- 

 tures occurs at the time mentioned.) 



