126 GENETICS IN RELATION TO AGRICULTURE 



tors. If the chromosome number is large, the chances of such experi- 

 ments showing factor linkage are slight. Finally there are experimental 

 difficulties in the way of securing an adequate body of data for most 

 animals and for practically all plants. It is necessary to conduct most 

 technical investigations in heredity with relatively meager financial sup- 

 port, consequently the expenditures necessary to obtain sufficient data 

 of this kind would be prohibitive for most of the larger animals and 

 plants. Moreover, on account of the time required to raise a sufficient 

 number of generations and to classify the individuals a considerable 

 time must elapse before a body of data can be gathered in any species 

 sufficient to submit it to the critical tests necessary to establish the 

 chromosome theory. Drosophila with its prolific breeding tendencies, 

 short life cycle, and ease of handling provides a form far superior to any 

 other thus far investigated for the elucidation of factor relations in general. 

 It is safe to say that our ideas of linkage for some time to come will be 

 largely determined by the results of the Drosophila investigations. 

 Particularly is this true because thus far none of the linkage phenomena 

 exhibited by other animals and by plants have yielded evidence contradic- 

 tory to the chromosome theory. The number of factors which have been 

 investigated in several species exceeds the number of pairs of chromo- 

 somes, nevertheless in no single case has there been a clear demonstra- 

 tion that the number of independently Mendelizing factors exceeds the 

 number of pairs of chromosomes. Moreover, those cases of linkage 

 which have been discovered are largely of factors for wholly unrelated 

 characters, just as in Drosophila. Added to this the ratios are of the same 

 diverse orders of magnitude and the linkage relations in general show no 

 essential difference from those which are displayed by Drosophila. It 

 would be nothing short of inconceivable, in fact, that the conclusions 

 reached from the Drosophila investigations are not applicable in all their 

 essential features to plant and animal forms in general. 



On the basis of the sweet pea and Primula investigations, the English 

 school of geneticists, represented particularly by Punnett and Trow, has 

 developed a theory of linkage very different from that outlined in this 

 chapter, which is called reduplication. According to this hypothesis 

 segregation occurs in a series of cell divisions preceding the reduction 



divisions, and for linked factors gives gametic series mostly of the form 



* 

 For coupling (n 1):1:1: (n 1) 



For repulsion 1: (n 1): (n 1): 1. 



In these ratios n is some power of two. Interaction of two such series 

 may give secondary reduplications which give different values for the 

 terms of the ratio. This theory of linkage cannot, however, lay claim 

 to the experimental support which the chromosome theory has obtained, 



