CHROMOMERES 271 



The problem then arises, how is it that the char- 

 acters are only partially, and not completely, coupled ? 

 Morgan's answer is that when the chromosomes unite 

 in pairs before the reduction division they do not 

 simply lie side by side, but become twisted round one 

 another, and that instead of untwisting and becoming 

 separated unchanged before the nuclear division, they 

 may break across, so that what was part of one chromo- 

 some becomes attached to what was part of the 

 other. 



If now the determiners for the characters are arranged 

 in series along the chromosome like the chromomeres 

 represented in Fig. 46, when two determiners are placed 

 close together, it is unlikely that the break will occur 

 between them, and such characters will be closely 

 coupled. But if they are placed farther apart the 

 break will more often come between them, and they 

 will more often become separated, so that the coupling 

 is less strong. And finally, if the determiners are 

 very far apart, the break will occur equally often in 

 such a way that they remain associated or become 

 separated, and in such a case there will be no 

 coupling. 



It should be mentioned that the twisting of chromo- 

 somes round each other has not been observed in 

 Drosophila, which is not well suited to cytological work, 

 but has been seen in other animals and plants.] 



The possibility still remains that in certain cases 

 particular characters may be associated with par- 

 ticular chromosomes as a whole, and we shall next 



