148 CHAPTER 18 



SUMMARY AND CONCLUSIONS 



The mutational events involving the largest amount of genetic material are changes in the 

 number of whole sets of chromosomes. The modes of origin and the breeding behavior of 

 autopolyploids are discussed, and the origin and structure of the giant chromosomes of 

 the salivary gland of Drosophila larvae are described. Ploidy can increase also via amphi- 

 ploidy. 



Polyploidy is more common among plants than among animals. When an individual 

 possesses an odd number of genomes, many of the gametes it produces have an incomplete 

 set in which a chromosome is more detrimental to survival when missing than when present 

 in excess. 



Chromosome addition or subtraction results in aneuploidy, and is a type of mutation 

 that produces too drastic a phenotypic change to be as important in evolution as euploid 

 changes in whole chromosome sets. 



Loss or gain of single whole chromosomes can be produced by means of nondisjunction 

 and the segregation of chromosomes in polyploids possessing an odd number of genomes. 

 Not only do such mutations occur in the germ and somatic lines spontaneously, but they 

 may be initiated or have their frequency enhanced by physical and chemical factors. 



REFERENCES 



Blakeslee, A. F., "New Jimson Weeds from Old Chromosomes," J. Hered., 25:80-108, 1934. 



Blakeslee, A. F., and Belling, J., "Chromosomal Mutations in the Jimson Weed, Datura 

 Stramonium," J. Hered., 15:194-206, 1924. 



Bridges, C. B., and Brehme, K. S., The Mutants of Drosophila Melanogaster, Washington, 

 D.C., Carnegie Institution of Washington, Publ. 552, 1944. 



Dobzhansky, Th., Genetics and the Origin of Species, 2nd Ed., New York, Columbia Uni- 

 versity Press, Chap. 7, pp. 223-253, 1941. 



Heitz, E., and Bauer, H., "Beweise fiir die Chromosomennatur der Kernschleifen in den 

 Knauelkernen von Bihio hortulanus L. (Cytologische Untersuchungen an Dipteren, I)," 

 Z. Zellforsch., 17:67-82, 1933. 



Painter, T. S., "A New Method for the Study of Chromosome Rearrangements and Plotting 

 of Chromosome Maps," Science, 78:585 586, 1933. Reprinted in Classic Papers in 

 Genetics, J. A. Peters (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 1959, pp. 161-163. 



Patau, K., Smith, D. W.,Therman, E., Inborn, S. L., and Wagner, H. P., "Multiple Congenital 

 Anomaly Caused by an Extra Autosome," Lancet, 1:790-793, 1960. 



White, M. J. D., Animal Cytology and Evolution, 2nd Ed., Cambridge, Cambridge University 

 Press, 1954. 



QUESTIONS FOR DISCUSSION 



18.1. How do we know that the genetic differences found in a population today were not 

 always present in it? 



18.2. From the present Chapter, what have you learned about the characteristics of mu- 

 tation? 



18.3. What is the relation between mutants and genes? mutants and recombination? 



18.4. From your present knowledge, how would you modify the statements on p. 21 relative 

 to the ploidy of gametes? 



18.5. Describe at least two different ways that the trisomy causing Mongolism may origi- 

 nate. 



