;st 



( II U'TER 30 



5. We have also noted that in Ascaris (p. 

 189) changes occurring in somatic tissues 

 lead to the formation of a number of small 

 chromosomes from a single large one. 



6. The frequency of nondisjunction lead- 

 ing to aneusomy has been shown to depend 

 both on the amount and distribution of 

 heterochromatin and on the types of chro- 

 mosomal rearrangements present. There- 

 fore, to the extent that the genotype regulates 

 its heterochromatin and rearrangements, it 

 is also regulating the incidence of nondis- 

 junction. 



7. Similarly, the arrangement of meiotic 

 products in Drosophila oogensis (Chapter 

 1 2 ) acts to eliminate dicentrics produced by 

 crossing over in paracentric-inversion hetero- 

 zygotes. 



8. Finally, the arrangement of the chro- 

 mosomal material and the metabolic activity 

 of the cell (as it influences the amount of 

 water and oxygen present, for example) are 

 other ways in which mutability is influenced 

 or regulated by the genotype itself. 



The preceding discussion dealt mainly 

 with the prevention or regulated occurrence 

 of intergenic changes. Does the genotype 

 regulate the occurrence of point mutation? 

 Consider the spontaneous point-mutation 

 frequencies for two lines of the same species 

 of Drosophila — one living in a tropical and 

 the other in a temperate climate. If the 

 genotype were at the mercy of temperature 

 in the wild, we would expect the tropical 

 form to have a higher frequency of spontane- 

 ous point mutation than the temperate form. 

 However, when both lines are grown at the 

 same temperature in the laboratory, the trop- 

 ical form has a lower mutation rate than 

 the temperate one. This result provides 

 good evidence that the tropical form has 

 genetically suppressed (or the temperate form 

 has genetically enhanced) its mutational re- 

 sponse to temperature. Consequently, in na- 

 ture the two forms probably show less differ- 



ence in mutation frequency than would be 

 expected with the differences in temperature. 

 Other strains of Drosophila melanogaster col- 

 lected from various regions have ditferent 

 spontaneous point-mutation frequencies. 

 Some o\' this may be due to differences in 

 the mutability of their isoalleles (p. 59); 

 part may be due also to a general control of 

 mutability by the genotype, for some strains 

 contain mutator genes which can increase the 

 general point-mutation frequency as much 

 as tenfold. Of course, other alleles of mutator 

 genes can be considered general suppresssors 

 of point mutability. Certain organisms 

 (bacteria, for example) have mutants which 

 make the individual generally less mutable to 

 a given mutagen. Since the organisms most 

 advanced in evolution contain more genetic 

 material per cell than less advanced forms, 

 the most advanced forms probably have se- 

 lected genotypes which reduce their sponta- 

 neous mutation rate to avoid overmutation. 



Activator and Dissociation in Maize 2 



The triploid endosperm (p. 26) in kernels 

 from some corn plants are white, others are 

 colored, and still others are white with col- 

 ored speckles. At first, it might seem as if 

 we were dealing with a high mutation fre- 

 quency of a gene from a "colorless" to a 

 "colored'' allele. It is found, however, that 

 the white phenotype results from the pres- 

 ence of two genes adjacent to or very near 

 each other on the same chromosome. If 

 these two loci are separated or dissociated 

 from each other by chromosomal breakage 

 that removes a particular one of the two loci, 

 the mutant cell and all its daughter cells with 

 the remaining locus will be colored. The 

 locus removed, called Dissociation, Ds, 

 causes breakage in chromosome regions near 

 it and is probably in a heterochromatic por- 

 tion of the chromosome. If Ds is never dis- 

 sociated from the adjacent locus, the kernel 



Based upon work of B. McClintock. 



