MEIOSIS 205 



arranged according to chance. In CE. novce-scotice, for example, 

 in which fourteen of the chromosomes are arranged in a single 

 ring, the total number of ways of arranging fourteen chromosomes 

 within a single ring is Z 13. The total number of ways of arranging 

 fourteen chromosomes so that alternate pairs pass to the opposite 

 poles in such a way that no two homologous chromosomes pass to 



/g /7 2'^ 



the same pole is '- '- — '. It therefore follows that if the 



2 



arrangement be haphazard within the ring, the chances of non- 

 disjunction taking place will be Z13 : Z6 . /7 . 2^, i.e., 429 : 16. 

 Thus, if fourteen chromosomes be arranged within the ring accord- 

 ing to pure chance, just over 3- 5 per cent, in the heterotypic 

 division of all homologues will pass to the opposite poles. There- 

 fore a large amount of functionless pollen will be produced and a 

 number of mutants thrown in each generation. Hence the arrange- 

 ment within the ring cannot be according to chance. In those 

 forms in which pairing and linkage of chromosomes is a regular 

 occurrence, the inheritance of characters borne by the homologous 

 pairs will take place probably according to normal Mendelian 

 lines. The joining of the chromosomes into large rings and the 

 ultimate segregation of those chromosomes which alternate is 

 essentially a new form of linkage, each linkage group acting as a 

 unit in heredity. When complete pairing occurs and linkage is 

 observed between certain morphological characters, the factors 

 responsible for these characters must be contained on a single 

 chromosome. However, when the manner of segregation at 

 anaphase is controlled by the linkage of a number of chromosomes, 

 the factors responsible for linked morphological characters need 

 not be situated on the same chromosome, but must be in the same 

 group of linked chromosomes. 



Abnormalities in the segregation of linked chromosomes 

 are of frequent occurrence at anaphase and arise through two 

 adjacent chromosomes of the spireme passing to the same 

 daughter nucleus. This may result in a six-eight distribu- 

 tion. If, however, the irregularity is to the same extent com- 

 pensated by two adjacent chromosomes passing to the opposite 

 pole, the normal number of chromosomes will go to each pole, but 



