CHAPTER 2 



The Modifications of the Chromosome Cycle 



Mendel's original experiments were concerned with factors which 

 are expressed in that part of the life cycle of the plant in which the 

 nucleus is diploid, containing two of each type of chromosome. Pre- 

 dictions can be made from the chromosome theory as to the ratios to 

 be expected for characters which are apparent in other phases of the 

 life cycle, and also as to the course of inheritance in organisms with 

 atypical nuclear cycles. These predictions form a valuable test of the 

 theory. We shall consider first the principles which apply to factors 

 which are expressed in various phases of the life cycles of plants, and 

 then the results of some different chromosomes cycles. The special case 

 of an abnormal chromosome cycle which is found in polyploids, i.e. in 

 organisms which possess three, four, or more of each kind of chromo- 

 some instead of the usual two of each kind, is of particular importance 

 and is considered in the last section of this chapter. 



A. CHROMOSOME CYCLES^ 



In animals the reduction divisions take place immediately before the 

 differentiation of the germ-cells, so that it is only in the gametes them- 

 selves that the nucleus is haploid. The haploid phase may, however, 

 assume much greater importance in plants. In some lower plants, e.g. 

 most fimgi and algae, the reduction division follows immediately after 

 fertilization, so that the nucleus is haploid throughout the life of the 

 organism with the exception of the just fertilized egg-cell; such organ- 

 isms are known as haplonts. In other plants the reduction division takes 

 place at some time between fertilization and gametogenesis ; in all 

 higher plants and some lower ones (e.g. Fucus) the interval between 

 reduction and gametogenesis is short, so that the haploid (gametophyte) 

 generation is relatively unimportant; but in many lower plants the 

 haploid phase which bears the gametes may be equal in duration and 

 morphological complexity to the diploid phase which occurs after 

 fertihzation and before reduction. The haploid cells produced by the 

 reduction division are known as spores (microspores in the male, macro- 



^ General references: Darlington 1937, 1939, Hartman 1929^? Stem 1928, 

 Wilson 1928. 



