6o MITOSIS : THE VARIATION OF THE CHROMOSOMES 



It follows that the chromosome difterences observed between 

 individuals and also between species must be interpreted in terms 

 of physiological conditions. For example, a difference in the size 

 of one chromosome may have much greater genetic and systematic 

 importance than a difference in the size of all the chromosomes in 

 the complement, for a general change may be determined by a 

 change in a single gene in one chromosome, if this happens to alter 

 the genetic properties of the organism in regard to chromosome 

 behaviour. Further, the individuality of the chromosomes is a 

 purely mechanical individuality. The individual chromosome has 

 no independent physiological control over its external form any 

 more than it has over anything else. The chromosome is a unit 

 statically as a structure seen at mitosis, dynamically as an organ of 

 segregation ; physiologically, it is merely part of the genotype to 

 which in many important properties it is subordinate. This 

 subordination may be likened to the subordination of members of a 

 legislature as individuals to the laws they enact as a body. 



It is therefore necessary to distinguish in all chromosome 

 behaviour between what is determined by the special properties of 

 the individual particles and what is determined by the genetic 

 reactions of the chromosomes as a whole, i.e., by the genotype. 

 This is particularly important for the analysis of the more complex 

 phenomena of meiosis, where genotypic control plays an even more 

 important part. 



3. NUMERICAL VARIATION 



(i) Classification of Changes. The characteristic group of chromo- 

 somes found at mitosis is spoken of as its complement. This com- 

 plement in a diploid is made up of two haploid sets. Reduplication 

 of some of the chromosomes of a set beyond the normal diploid 

 number is called polysomy, reduplication of the whole set so that 

 the nucleus contains three, four or more sets is called polyploidy. 

 The two combined give secondary polyploidy (D. and Moffett, 1930). 

 Any of these can result from abnormalities of (i) mitosis, (ii) meiosis, 

 (iii) fertilisation. 



Polyploidy in itself involves no change in the numerical 

 proportions of the chromosomes in the set ; it is said to be a 



