Chapter *25 



THE GENETIC CONTROL 

 OF MUTABILITY 



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N SEVERAL Chapters preceding 

 this, we have become famiHar 

 .with the characteristics of differ- 

 ent units of mutation, ranging from the larg- 

 est changes, genomic changes, to the smallest, 

 gene changes. We have seen, moreover, that 

 various externally applied environmental 

 agents can produce mutations of all kinds. 

 What is the nature of the role that the geno- 

 type plays in mutability? A few moments of 

 reflection will lead you to realize that in 

 several respects the very nature of the geno- 

 type influences its mutability. 



The fact that mitosis and meiosis occur in 

 the precise way they do is evidence that the 

 genotype normally prevents genomic and 

 single whole chromosome changes in succes- 

 sive generations of cells and individuals, re- 

 spectively. There can be no doubt that these 

 processes are under genie control — even if 

 we cannot specify in each case, or in any 

 particular case, the specific genes responsible 

 for these orderly mechanisms for mutation- 

 prevention. The synthesis of new genes must 

 usually be so orderly as to prevent improper 

 gene components from substituting for the 

 proper ones, on the presumption that both 

 types are present in the cell at the same time. 

 The process of synapsis between homologous 

 loci is adequately specific so that a chiasma 

 typically occurs at precisely corresponding 

 points of the two nonsister strands, and 

 crossovers resulting in deficiencies and dupli- 

 cations are avoided. (We have already men- 

 tioned some evidence for the genetic control 

 213 



of synapsis in the existence of coUochores, in 

 Chapter 21, and in the case of asynaptic 

 maize in Chapter 23.) 



It might be argued, however, that such ex- 

 amples only serve to demonstrate that the 

 processes mentioned reduce mutability as an 

 inevitable consequence of their normal opera- 

 tion. While it would be true in these cases 

 that the present genotype appears to play a 

 passive role, it must be admitted, since mitosis 

 and meiosis are not intrinsic properties of 

 genes, that during the course of evolution, 

 the selection of genes to carry out these ac- 

 tivities was an active process aimed at re- 

 ducing mutability, or, in other words, aimed 

 at maintaining genetic stability, while per- 

 mitting replication and genetic recombina- 

 tion via sexuality. 



While the genetic controls so far mentioned 

 lead to a reduction in mutability, it should be 

 recognized that the genotype also permits 

 genetic changes to occur in controlled or regu- 

 lated ways. This is demonstrated by the 

 very process of sexuality, whose ploidy 

 changes, from diploid to haploid and back 

 again, must be under genetic control. Since 

 mutational changes increase with mitotic 

 activity (Chapter 23), and because mitosis 

 and mitotic rate are known to be under 

 genetic control (many cancer cells are mu- 

 tants whose mitotic rate is increased), the 

 genotype controls mutability in this way. 

 We have already mentioned (Chapter 18) 

 certain modifications of meiosis, doubtless 

 also under genie control, which lead to 

 ploidy changes in the next generation. Even 

 within the somatic tissues of a multicellular 

 organism, controlled genetic change is per- 

 mitted in cells whose chromosomes become 

 polyploid (as in human liver) or polytene (as 

 in the Dipteran salivary gland). We have 

 also already noted (Chapter 23) in Ascaris 

 that changes from bipolarity to unipolarity 

 occur in somatic tissues, as a result of which 

 a number of small chromosomes are formed 

 from a single large one. (The genotype sup- 



