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THE AMERICAN NATURALIST [Vol. L 



lary to this theory. It has already been stated that he had 

 found different degrees of linkage to exist between the 

 various factors of a group : for example, the proportion 

 of cases in which separation occurs between white and 

 rudimentary was said to be 42 per cent., whereas the fre- 

 quency of separation between white (eye color) and the 

 factor for yellow body color is only about 1 per cent. In 

 explanation of these different degrees of linkage, Morgan 

 pointed out that, on the chiasmatype theory, the closer the 

 proximity of two factors to each other in the chromosome, 

 the smaller would be their frequency of separation, for 

 the less would be the chance for a crossing-over of the 

 chromosomes to occur between them. Thus, in Fig. 2, the 

 factors A and C become separated both in case (a) and 

 in case (b), because A and C lie so far apart that in both 

 cases the point of crossing-over falls between them, but in 

 only one of the cases do A and B separate, and in one case 

 B and C, since these are so near together that the point of 

 crossing-over may often be beyond instead of between 

 them. In other words, on the chiasmatype theory, fre- 

 quency of recombination must be, to a certain extent at 

 least, an index of the distance apart of factors along the 

 chromosome. Since the time when these ideas were pro- 

 posed (1911), two important series of facts have come to 

 light in the studies on Drosophila, in support of the chias- 

 matype theory of interchange and of these extensions of it. 



III. A Verification of the Theory of Crossing-Over. 

 The Law of Linear Linkage 

 It occurred to Sturtevant in 1911 that, if the factors 

 are carried in the chromosomes, then, owing to their linear 

 arrangement, the distance along the chromosome between 

 any two factors (A and C) must be either the sum or the 



A C _jg_3 A B C 



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