I7(> « HAP1 EB 12 



SUMMARY AND CONCLUSIONS 



Structural change in chromosomes is a type o! mutation involving the gain, loss, or 

 relocation of chromosome parts. All such mutations require chromosome or chromatid 

 breakage. Since proximitj favors union, most of the ends produced hy breakage 

 restitute. Unions occur mainly during interphase. Noniestitutional unions produce 

 structural changes ill chromosomes. The occurrence of one. two. or three nonrestituting 

 hreaks in one or two chromosomes is discussed in relation to the production of whole- 

 chromosome losses, deficiencies, duplications, inversions, translocations, shifts, and 

 transpositions. 



Chromosomes that have undergone structural change may be euploid or aneuploid. 

 The cells in which these mutations arise are euploid but can become aneuploid follow- 

 ing mitosis, segregation, or crossing over. The structural changes most likely to be 

 retained in the population are the smallest ones; those changes which directly or in- 

 directly cause an increase in gene number are most likely to be important in evolution. 



REFERENCES 



Beam. A. G.. and German III, J. L., "Chromosomes and Disease," Scient. Amer., 

 205. No. 5:66-76, 1961. 



Muller. H. J.. 'The Nature of the Genetic Effects Produced by Radiation," in Radiation 



Biology, Hollaender, A. (Ed.), New York: McGraw-Hill, 1954, Chap. 7, pp. 



351-473. 

 Patau. K., "The Origin of Chromosomal Abnormalities," Pathologie-Biologie, 11:1163- 



1 170, 1963. 

 Russell, L. B.. "Chromosome Aberrations in Experimental Mammals," Progress in 



Medical Genetics, 2:230-294, 1962. 



QUESTIONS FOR DISCUSSION 



12.1. The terms euploid and aneuploid (hypo- or hyperploid) have been applied both 

 to individual chromosomes and to whole nuclei. Give an example of: 



(a) A hypoploid chromosome in a euploid nucleus 



(b) A hyperploid chromosome in a hyperploid nucleus 



(c) An aneuploid nucleus containing all structurally normal chromosomes 



12.2. As used on p. 170, what does the term eutelomeric mean? Name two types of 

 aneutelomeric chromosomes. 



12.3. Given the chromosome AB/CDE/F.GHI/J, where the period indicates the 

 centromere and the slanted lines the positions of three simultaneously produced 

 breaks, draw as many different outcomes as possible. Indicate which one is 

 most likely to occur. 



12.4. In Drosophila, the loss of a given chromosome results in monosomy; this situa- 

 tion is approximately three to five times as frequent as its gain, resulting in 

 trisomy. Explain. 



12.5. Discuss the origin of monosomies among human zygotes. 



