144 GENETICS 



This proportion In which exchange has occurred Is known 

 as the exchange ratio, or as the cross-over ratio. This ex- 

 change ratio plays a most Important role in the evidence 

 on the questions with which we have to deal. 



Constancy of the Exchange Ratio for a Given Pair of 

 Characteristics: Approximately this same exchange ratio 

 Is produced whenever this particular mating Is made in a 

 large number of cases. If by such matings a thousand sons 

 are produced, and this is repeated, In each looo there will 

 be about 985 to 988 showing the original combinations, 

 about 12 to 15 showing the new combinations. If the orig- 

 inal parents show the reverse combinations (Red-Yellow 

 and White-Gray), then the new combinations Red-Gray 

 and White-Yellow will appear In about 12 to 15 cases out 

 of each 1000. Thus for a given pair of characters (as 

 White-Yellow) the exchange rate Is approximately con- 

 stant. 



Diverse Exchange Ratios in Different Pairs of Charac- 

 teristics: If In place of the characteristics we have been con- 

 sidering, some other pair of recessive characteristics Is 

 crossed with normals, a different constant exchange ratio 

 Is produced. Thus, If individuals with yellow body and bifid 

 wings (characters connected with the X-chromosome) are 

 mated with normal individuals (having gray body and nor- 

 mal wings), and the second generation Is obtained In the 

 way Illustrated In figure 32, the sons show an exchange 

 ratio of about 5.3 per cent. And this ratio is produced, ap- 

 proximately, whenever this pair of characteristics is tested. 



A Series of Diverse Exchange Ratios: Still other pairs 

 of characteristics give still different exchange ratios. If 

 matings are made using many different pairs, many differ- 

 ent exchange ratios are produced. It Is particularly instruc- 

 tive to examine the ratios for a series of pairs, one member 

 of which is the same in each case. Taking white eyes as the 

 member that is the same In each case, the folloAving series 



