18 CALVIN B. BRIDGES 



The development of the idea of interference is an illustration 

 of the advantages of the chromosome hypothesis. The existence 

 of this phenomenon was originally deduced by Muller and 

 Sturtevant from a consideration of linkage as a chromosome 

 process. 



Linkage had been explained as the result of two loci in the 

 same chromosome being so close together that a chiasma occurs 

 between them infrequently. If the chiasma must occur at a 

 node, then, since the internode has considerable length, there 

 must be on either side of the chiasma space which is free from 

 crossing-over, A couple of gens C-D might lie at such a dis- 

 tance from A~B that when a chiasma occurred between A and .6, 

 the length of the internode would cause the next chiasma to 

 occur most often to one side of C-D rather than between C and D. 

 In any one maturing egg a chiasma between the members of 

 one such couple would tend to prevent one between the members 

 of the other couple; consequently, few gametes would be formed 

 which would be the result of both occurring simultaneously, 

 that is, douhle-crossing-over loould he interfered with. While 

 the phenomenon of interference is thus a corollary of the chromo- 

 some hypothesis, it is almost unexplainable upon any other view 

 of linkage. 



Since the accuracy of the calculation of the amount of inter- 

 ference depends upon the accuracy of the smallest class — the 

 double crossovers — in practice we derive the index of inter- 

 ference from a comparison of the observed percentage of double 

 crossovers with the percentage which would be expected if 

 there were no interference. The percentage of double cross- 

 overs expected without interference is the product of the total 

 percentage of crossing-o^'er between A and B by the like value 

 for C and D. That is, if there is 5 per cent of crossing-over 

 between A-B, and 10 per cent between C-D, then 5 per cent of 

 10 per cent would give the percentage of cases in which both 

 occur. The case is exactly similar in treatment to the 9:3:3:1 

 rat'o of two freely assorting pairs of gens, where 25 per cent of 

 all cases are a and 25 per cent are h, and 25 per cent of 25 per 

 cent gives the percentage occurrence of ah. In order to obtain 



