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 asin 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: (mn — 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, 
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