292 CHAPTER 32 



SUMMARY AND CONCLUSIONS 



The general hypothesis "one cistron-one primary effect" states that a cistron produces only 

 one primary effect and that any primary effect is the result of the action of a single cistron. 



The specific hypothesis, one enzyme-one cistron, proposed as a test of the general hy- 

 pothesis, was found to be essentially correct. However, its modification was necessitated 

 by studies of the biochemical genetics of tryptophan synthetase and hemoglobin. These 

 studies required that the specific hypothesis be made more general, so that it is stated as 

 "one polypeptide-one cistron," and means that each polypeptide is specified completely 

 by the primary action of a single cistron. 



It is concluded that one way a cistron can act in a primary way is to specify the amino 

 acid content of a polypeptide. 



It is hypothesized that a cistron is composed of a number of linearly arranged recons. 



REFERENCES 



Beadle, G. W., and Tatum, E. L., "Genetic Control of Biochemical Reactions in Neuro- 

 spora," Proc. Nat. Acad. Sci., U.S., 27:499-506, 1941; reprinted in Classic Papers in 

 Genetics, Peters, J. A. (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 1959, pp. 166-173. 



Harris, H., Human Biochemical Genetics, Cambridge University Press, 310 pp., 1959. 



Hsia, D. Y.-Y., Inborn Errors of Metabolism, Chicago, Year Book Publishers, 1959. 



Ingram, V. M., "How Do Genes Act?" Scient. Amer., 198:68-74, 1958. 



Itano, H. A., and Robinson, E. A., "Genetic Control of a- and /3-Chains of Hemoglobin," 

 Proc. Nat. Acad. Sci., U.S., 46:1492-1501, 1960. 



Yanofsky, C, and Crawford, I. P., "The Effects of Deletions, Point Mutations, Reversions 

 and Suppressor Mutations on the Two Components of the Tryptophan Synthetase 

 of Escherichia CoH," Proc. Nat. Acad. Sci., U.S., 45:1016-1026, 1959; reprinted in 

 Papers on Bacterial Genetics, Adelberg, E. A. (Ed.), Boston, Little, Brown, 1960, 

 pp. 384-394. 



See Supplement IV and the first portion of Supplement V. 



QUESTIONS FOR DISCUSSION 



32.1. What significance can you give to the fact that it is a glutamic acid in hemoglobin A 

 which is replaced by another amino acid (valine, lysine, or glycine) in hemoglobins S, 

 C, G, and E? 



32.2. What are the disadvantages of human beings as material for the investigation of the 

 cistron? 



32.3. Can crossing over occur within a gene? Explain. 



32.4. Is the hypothesis, one cistron-one primary function, equivalent to the hypothesis, 

 one polypeptide-one cistron? Why? 



32.5. What evidence can you give for rejecting the hypothesis that a cistron is equivalent 

 to a single recon? 



32.6. Would you expect that a chemical substance, specified in a primary way by a cistron, 

 would be composed of linearly arranged parts? Why? 



32.7. Can you apply the term cistron to the one or more recons that determine whether 

 glutamic acid or lysine shall be located at a particular place in hemoglobin? Why? 



32.8. What things do genes actually do? Has your answer any bearing upon the concept 

 of a cistron? Explain. 



32.9. Under what circumstances is it proper to use the term allele to describe cistronic 

 alternatives rather than reconic alternatives? 



