CHAPTER V 

 THE MECHANISM OF ASSORTMENT 



Each pair of chromosomes, just before the reduction 

 division, consists of a maternal and a paternal member. 

 As the members of each pair are in nearly all cases identi- 

 cal in appearance, it is not possible to tell how they place 

 themselves on the mitotic spindle with respect to their 

 parental origin ; that is, it is not possible to tell by inspec- 

 tion whether at the maturation division all those of mater- 

 nal origin pass to one pole of the spindle, and all those of 

 paternal origin to the other, or whether the pairs come to 

 lie haphazard on the spindle, so that it is merely a matter 

 of chance whether a maternal or a paternal member passes 

 to a particular pole. For the utilization of the chromo- 

 somal mechanism for the theory of assortment, it is a 

 matter of great importance which of the preceding alter- 

 natives is followed, for if all of the maternal chromosomes 

 should go to one pole, and all the paternal to the other, 

 there would be no free assortment of the chromosomes, 

 and no free assortment of the genes if these are carried by 

 the chromosomes. Without random assortment there 

 could only be two kinds of gametes produced by the 

 hybrid, hence only three types possible in F2, viz., the two 

 grandparental types and the hybrid type. 



On the other hand, if the assortment of chromosomes 

 is a random one, then the reduction division furnishes the 

 mechanism that MendePs law calls for in so far as the 

 character-pairs lie in different chromosome pairs. 



There is not a single cytological fact opposed to the 

 view of free assortment of maternal and paternal chromo- 

 somes; on the contrary, there is a general expectation 

 that the chromosomes should assort freely; and what is 

 more to the point, there are a few crucial cases that show 

 that free assortment takes place. Let us turn to these 



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