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CHAPTER 25 



presses mutation in a polycentric chromo- 

 some by suppressing the action of all but one 

 centromere.) The frequency of nondisjunc- 

 tion, which leads to aneusomy, has been 

 shown to be dependent upon both the 

 amount and distribution of heterochromatin, 

 and also upon the types of chromosomal 

 rearrangements present. So, insofar as the 

 genotype regulates its heterochromatin and 

 rearrangements, it is also regulating the in- 

 cidence of nondisjunction. Similarly, the 

 arrangement of meiotic nuclei in the oogenesis 

 of Drosophila (Chapter 19) eliminates di- 

 centrics produced by crossing over in para- 

 centric inversion heterozygotes. Finally, the 

 arrangement of the chromosomal material 

 and the metabolic activity of the cell (amount 

 of water and oxygen present, for example) 

 are other ways in which mutability is regu- 

 lated by the genotype itself. 



The preceding discussion has dealt largely 

 with the prevention or regulated occurrence 

 of intergenic changes. Is there other, specific 

 evidence that the genotype regulates the oc- 

 currence of point mutation? Compare the 

 spontaneous point mutation rates of two 

 lines of the same species of Drosophila, one 

 living in a tropical and the other in a temper- 

 ate climate. If the genotype was at the mercy 

 of temperature, in nature, the tropical form 

 would be expected to have a higher rate of 

 spontaneous point mutation than the tem- 

 perate form. However, when both lines are 

 grown in the laboratory at the same tempera- 

 ture, the tropical form has a lower mutation 

 rate than the temperate one. This is good 

 evidence that the tropical form has genetically 

 suppressed (or the temperate form has geneti- 

 cally enhanced) its mutational response to 

 temperature. Accordingly, the two forms, in 

 nature, probably show less difference in mu- 

 tation rate than expected from the tempera- 

 ture difference. Other strains of Drosophila 

 melanogaster collected from different regions 

 have different spontaneous point mutation 

 rates. Some of this difference may be due to 



differences in mutability of the isoalleles 

 (cf. p. 65) that are present and which result in 

 the same wild-type appearance in each case; 

 but part may be due also to a general control 

 of mutability by the genotype. For some 

 strains are known to contain mutator genes 

 in whose presence the general point muta- 

 tion rate is increased as much as tenfold. Of 

 course, other alleles of mutator genes may 

 therefore act as general suppressors of point 

 mutability. Certain organisms (bacteria, for 

 example) have mutants which make the indi- 

 vidual generally less mutable when exposed 

 to a given mutagen. Note also that the or- 

 ganisms most advanced in evolution contain 

 more chromosomal material per cell than do 

 less advanced forms, and they have probably 

 selected genotypes that reduce their sponta- 

 neous mutation rate to avoid overmutation. 



Consider next certain results in maize. ^ 

 The kernels of some plants are white, others 

 are colored, while still others are white with 

 colored speckles. At first, it appears as 

 though we are dealing with a high rate of 

 mutation of the "colorless" gene. It was 

 found, however, that the white phenotype is 

 the result of the presence of two genes adja- 

 cent to or very near each other on the same 

 chromosome. If these two loci become dis- 

 sociated from each other, as when a particu- 

 lar one is removed via a two-break deletion, 

 for example, the mutant cell and all its 

 daughter cells containing the remaining locus 

 are colored. The locus removed is called 

 Dissociation, Ds; this locus can be the cause 

 of breakage in chromosome regions near it, 

 and was shown to be a heterochromatic por- 

 tion of the chromosome. If Ds is never dis- 

 sociated from the adjacent locus, the kernel is 

 white ; if it is dissociated during kernel forma- 

 tion, the kernel has colored sectors or dots on 

 a white background; if Ds is moved before 

 the kernel forms, the kernel and later genera- 

 tions of plants are completely colored. The 

 mutation described here is the loss or removal 

 ^ Based upon work of B. McClintock. 



