Point Mutations 199 



H. J. Muller, at Cold Spring Har- 

 bor, N.Y., 1941. 



protraction and no threshold dose below which the genetic material is safe from change. 

 Point mutations also indicate that a given gene is relatively stable over many cell gen- 

 erations — changes in genes resulting from very localized physico-chemical events last- 

 ing a matter of minutes, after which the new gene is stable. Point mutations are en- 

 hanced or induced by temperature changes, aging, gene replication, and physical and 

 chemical mutagens. It is possible that changes resulting in point mutants take place 

 in the old gene, in the new gene, or during the formation of the new gene. 



Genetic schemes for the detection of X-linked recessive lethal and recessive visible 

 mutants in Drosophila are described. A single representative of most normal genes 

 fails to produce the full normal phenotypic effect, and most point mutants act on the 

 phenotype in a hypomorphic or amorphic manner. The study of point mutants of 

 these and other types reveals that almost all are detrimental to the reproductive poten- 

 tial of individuals when pure (not hybrid), and to a lesser extent when hybrid. Ac- 

 cordingly, most point mutants are not completely recessive to their normal genetic 

 alternatives. 



REFERENCES 



Alexander, P., "Radiation-Imitating Chemicals," Scient. Amer., 202. No. 1:99-108, 

 1960. 



Crow, J. F., and Temin, R. G., "Evidence for Partial Dominance of Recessive Lethal 

 Genes in Natural Populations of Drosophila," Amer. Nat., 98:21-33, 1964. 



Muller, H. J., "Variation Due to Change in the Individual Gene," Amer. Nat., 56:32- 

 50, 1922. Reprinted in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood 

 Cliffs, N.J.: Prentice-Hall, 1959, pp. 104-116. 



Muller, H. J., "Artificial Transmutation of the Gene," Science, 66:84-87, 1927. Re- 

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

 Prentice-Hall, 1959, pp. 149-155, and also in Great Experiments in Biology, 

 Gabriel, M. L., and S. Fogel (Eds.), Englewood Cliffs, N.J.: Prentice-Hall, 1955, 

 pp. 260-266. 



Muller, H. J., "A Semi-automatic Breeding System ('Maxy') for Finding Sex-linked 

 Mutations at Specific 'Visible' Loci," Drosophila Info. Serv., 28:140-141, 1954. 



