138 



Shull. 



Xa hC : 



chaiiare places, each uniting with the homolog of the other, as illustrated 

 diagraininatically in fig'. 6, the result would be exactly the same as 

 if two determiners of the same type had originated independently in 

 these two pairs of chromosomes, for now two chromosomes (3 and 4, 

 fig. 6), each bearing the gene C, could enter into a single germ-cell, 

 while into the sister germ-cell would pass two chromosomes in which 

 tiie same determiner is absent. This same chromosome might cany 

 other determiners in a coupled system with C, all acting together os a 

 single unit, liut it matters not in such a case how complicated the par- 

 ticular chromosome is, — over how 

 large a portion of the organism it 

 operates, or in how many different 

 l)hysiological and morphological fea- 

 tures it produces characteristic visible 

 or invisible effects, — the duplicate de- 

 terminer would represent exactly the 

 same coupled system and play exactly 

 the same role, because, although located 

 in a new position this chromosome 

 would really be the same organ and 

 not a new and independently oi-iginated 

 duplicate of it. 



There is another readily conceiv- 

 able method of rearrangement of genes 

 by which a du])lication of determiners 

 would be brought about. While it is 

 now known that chromosomes do not 

 invariably unite to form a continuous 

 spireme, the work of numerous cytol- 

 ogists has tuade it proliable that this 

 is the usual procedure. At one stage 

 in cell-division tlie chromosomes are united into an apparently continuous 

 strand, and at another stage the strand segments preparatory to tlio 

 formation of the daughter chromosomes. If the determiner C haii|HMird 

 to be located in the extremi' end of one chromosome (.3,3, fig. 7). it 

 seems a very reasonable assumittion that the break in the strand wliicli 

 forms the new chromosomes might once occur on the opposite side of 

 this determiner, so that instead of lying in the chromosome in which 

 it had been originally located (3), it would become a part of the reverse 

 end of thr adjacent chromosome (4). The same result might come nbont 



Fig. 0. Hypothetical duplication of a 

 determiner by the displacement of a 

 chromosome. X represents the unhy- 

 pothesized residual constitution of the 

 chromosome; A. B, C. are hypothetical 

 Mendelian genes; a. b, c, simply call 

 attention to the absence of the genes rep- 

 resented by the corresponding capitals. 



