Structural Changes Within Chromosomes 151 



The chromosomes that have undergon estructural change may be euploid or aneuploid. 

 The cells in which these mutations arise are euploid but may become aneuploid following 

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

 the population are the smallest ones, those directly or indirectly causing an increase in gene 

 number being 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, A. Hollaender (Ed.), New York, McGraw-Hill, 1954, Chap. 7, pp. 351-473. 



QUESTIONS FOR DISCUSSION 



19.1. 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. 



19.2. Could a reciprocal translocation occur between homologous chromosomes? Explain. 



19.3. What does the term eutelomeric mean, as used on p. 156? 



19.4. While most Mongolian idiots are trisomic for a particular small autosome, some are 

 known that have 46 chromosomes. In the latter cases, all chromosomes seem of 

 normal composition except that one of the homologs of a different autosome has 

 one arm that is exceptionally long. Discuss the origin and cause of Mongolian 

 idiocy in these exceptional cases. 



19.5. Given the chromosome AB CDE/F.GHI/J, where the period indicates the centro- 

 mere and the slanted lines the positions of three simultaneously produced breakages, 

 draw as many different outcomes as you can. Indicate which one is the most likely 

 to occur. 



19.6. The loss of a given chromosome in Drosophila, resulting in monosomy, is approxi- 

 mately 3-5 times as frequent as its gain, resulting in trisomy. Explain. 



19.7. Discuss the frequency of monosomies among human zygotes. 



19.8. Discuss the detectability, in human chromosomes at mitotic metaphase, of a: 



a. Paracentric inversion. 



b. Pericentric inversion, 



c. Deficiency. 



d. Duplication. 



e. Half-translocation. 



19.9. What advantages may inversion provide? 



19.10. What characteristics of cells undergoing oogenesis favor the production and viable 

 transmission of half-translocations? 



19.11. In Drosophila, a male, dihybrid for the mutants hw and st, when backcrossed to 

 bw hw St St, normally produces offspring whose phenotypes are in a 1:1:1:1 ratio. 



On exceptional occasions, this cross produces offspring which are clearly of only 

 two of the four phenotypes normally obtained. How can you explain such an 

 exception? 



19.12. Is the telomere a gene? Why? 



