CHAPTER 5 



BASES OF CHANGE 



Polyploidy and Polysoiiiy Structural Cliaugc Misd'wision oftiie Cciilroiiwrc 



Deficieticy and Biilancc Intergeuic and Ititragcnic Clmiigc: 



Presence and Absence Somatic Mutation 



The Gene as a Unit oj Clianqe 



So FAR WE HAVE CONSIDERED experiments in which genes have 

 appeared as fixed in the chromosome, and chromosomes, apart from 

 crossing-over, as fixed in the nucleus. In any large experiment, 

 however, we find individuals which do not correspond to the 

 predictions that we are justified in making on the basis of the simple 

 rules of segregation and recombination. We fmd new and unex- 

 pected phenotypes. The^e variants, sports or mutants have changed in 

 their heredity. What kinds of changes occur and how do they come 

 about ? To answer these questions we call on a variety of methods 

 and observations. 



Polyploidy and Polysomy 



If we cut down a plant of the tomato, Lycopersicum escidentum, 

 a proportion of the shoots which subsequently grow from the cut 

 surface are of somewhat stouter growth. They have larger flowers, 

 but the ripe fruits are smaller with fewer seeds. These shoots, which 

 can be propagated as cuttings, and from their seeds, have 48 chromo- 

 somes instead of the usual 24. They are tetraploid with four sets of 

 chromosomes (2n = 4x) instead of diploid (2n = 2x). 



Doubling of the whole nucleus arises from a failure of mitosis 

 to complete itself: the chromosomes have divided without the 

 daughter nuclei separating and without the cell dividing. This 

 fiilure is very common in plants and can be readily induced by 

 treatment of the seed, the growing point of the shoot, the young 

 embryo, or the germ mother-cells with colchicine, with high or 

 low temperatures, and in other ways. Tetraploids have larger cells 

 and are generally larger and more robust than their diploid forbears. 

 Their fertility is usually reduced. Otherwise their resemblance to 



95 



