8 



Cytogenetics 



A MAJOR corollary of the chromosome theory of heredity is that changes 

 in number, structure, or behavior of chromosomes must represent, to 

 some extent, changes in the genetic potential of the organisms involved. 

 The study of the correlation between cytologically observable chromo- 

 some change and genetic change constitutes the hybrid discipline of 

 cytogenetics. The present chapter is concerned with those chromosome 

 changes which are of particular significance in this regard. These include 

 increase and decrease in chromosome number, interchanges between 

 chromosomes, and rearrangements within chromosomes. 



The basic organism in genetic theory is the true diploid, which may be 

 defined as one possessing two chromosome sets, the chromosomes of one 

 set being of maternal, the other of paternal origin. The genes associated 

 with the chromosomes making up each set constitute a genome; there- 

 fore the diploid organism contains two matching genomes, or sets of 

 genes, arranged on two matching sets of chromosomes. The chromosome 

 constitution of an organism may be altered either by spontaneous or 

 induced "accidents." These changes are classifiable and may be related, 

 in theory at least, to their origins. 



CHANGES IN CHROMOSOME NUMBER 



Polypioidy 

 The simplest and perhaps the most important change in chromosome 

 constitution involves the duplication of one or more complete chromo- 

 some sets (Figure 8-1 (a) and (b)) to give a polyploid organism. A 



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