1S2 CHAPTFR 38 



SUMMARY AND CONCLUSIONS 



Phenotypic changes can result when the same genetic material is arranged in different 

 ways. 1 he shuffling ol genes which produces such position effects may he brought 

 about b\ structural changes in chromosomes and by crossing over. 



linear nests of genes with similar effects have probably arisen by one or more 

 duplications in situ o! an ancestral gene, followed by mutations that led to differentiation 

 in their effects. 



Position effect is attributed to a change in one or more of the following: 



1. Production of a given messenger RNA 



2. Translation oi this messenger RNA 



3. Interactions between proteins resulting from the intranuclear translation of mes- 

 senger RNAs. 



REFERENCES 



Bridges. C. B.. "The Bar "Gene" a Duplication." Science. 83:210-211, 1963. Reprinted 

 in Classic Papers in Genetics. Peters. J. A. (Ed.). Englewood Cliffs, N.J.: Prentice- 

 Hall. 1959, pp. 163-166. 



Chovnick. A.. Schalet. A.. Kernaghan. R. P., and Krauss. M., "The Rosy Cistron in 

 Drosophila melanogaster: Genetic Fine Structure Analysis." Genetics, 50:1245- 

 1259. 1964. 



Lewis. E. B.. "The Pseudoallelism of White and Apricot in Drosophila melanogaster," 

 Proc. Nat. Acad. Sci., U.S., 38:953-961. 1952. 



Lewis. E. B.. "Genes and Developmental Pathways." Amer. Zool., 3:33-56, 1963. 



Muller. H. J., Prokofyeva-Belgovskaya. A. A., and Kossikov, K. V.. "Unequal Cross- 

 inszover in the Bar Mutant as a Result of Duplication of a Minute Chromosome 

 Section." C. R. (Dokl.) Acad. Sci.. U.R.S.S., N.S.. 1 ( 10) :87-88, 1936. 



Peterson. H. \L. and Laughnan. J. R.. "Intrachromosomal Exchange at the Bar Locus 

 in Drosophila." Proc^Nat. Acad. Sci., U.S.. 50:126-133, 1963. 



Sturtevant. A. H.. "The Effects of Unequal Crossingover at the Bar Locus in Dro- 

 sophila," Genetics, 10:117-147, 1925. Reprinted in Classic Papers in Genetics, 

 Peters. J. A. (Ed.), Englewood Cliffs, N.J.: Prentice-Hall, 1959, pp. 124-148. 



Welshons. W. J., and Von Halle. E. S., "Pseudoallelism at the Notch Locus in Dro- 

 sophila." Genetics. 47:743-759. 1962. 



QUESTIONS FOR DISCUSSION 



38.1. If a previously unknown phenotype appears at the same time as a qualitative 

 or quantitative change in the genetic material, can you determine whether the 

 effect is due to mutation or to position effect? Explain. 



38.2. Would you expect to find position effects in most sexually reproducing organ- 

 isms? Why? 



38.3. Is crossing over ever unequal? Explain. 



38.4. Should pseudoalleles be considered subgenes (parts of one gene) rather than 

 separate, nonallelic genes? Explain. 



38.5. Does position effect require pseudoallelism for its detection? Explain. Is the 

 reverse true? Explain. 



