22(5 CHAPTER 16 



SUMMARY AND CONCLUSIONS 



Cross-fertilizing species carrj a large load oi mutants in heterozygous condition. The 



vasl majorit) ol them are detrimental when homozygous and to a lesser extent — when 

 heterozygous, although some heterozygotes are superior to either homozygote. Other 



things being equal, almost all mutants are harmful to the same degree in that each 

 eventually causes genetic death. Mutants producing the smallest detriment to repro- 

 ductive potential cause the greatest total amount of suffering. More detriment and 

 more genetic deaths occur in heterozygotes than in homozygotes for rare mutants. 

 Persistence of a mutant in the population is inversely related to its selection coefficient. 



Mutation is the current price paid by a population for the possibility of having a 

 greater reproductive potential in the same or a different environment in the future. 

 So, despite the rarity of mutants which increase reproductive potential in a given en- 

 vironment, mutation provides the raw materials for evolution. 



Natural and man-made penetrating radiations are undoubtedly causing mutations 

 in our somatic and germ cells, increasing our load of detrimental mutants. This 

 exposure, though harmful, is most likely no threat to man's survival as a species at 

 present, although it might be in the future should the exposure become large enough. 

 Further research is needed to accurately assess the effects of high-energy radiations 

 and chemical substances upon man's mutation rate and well-being. 



REFERENCES 



Auerbach, C, Genetics in the Atomic Age, Fair Lawn, N.J.: Essential Books, 1956. 



Background Material for the Development of Radiation Protection Standards, Report 

 No. 1, Federal Radiation Council, Washington, D.C.: U.S. Government Printing 

 Office, 1960. 



Chu, E. H. Y., Giles, N. H., and Passano, K., "Types and Frequencies of Human Chro- 

 mosome Aberrations Induced by X-rays," Proc. Nat. Acad. Sci., U.S., 47:830-839, 

 1961. 



Crow. J. F., "ionizing Radiation and Evolution," Scient. Amer., 201:138-160, 1959. 



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. 



Dobzhansky, Th., Evolution, Genetics, and Man, New York: John Wiley & Sons, 1955. 



Dobzhansky. Th., "How Do the Genetic Loads Affect the Fitness of Their Carriers in 

 Drosophila Populations?" Amer. Nat., 98:151-166, 1964. 



Herskowitz, I. H.. "Birth Defects and Chromosome Changes," Nuclear Information, 

 3 (No. 2): 1-2, 4, 1960. 



Krieger, H., and Freire-Maia, N., "Estimate of the Load of Mutations in Homogeneous 

 Populations from Data on Mixed Samples," Genetics, 46:1565-1566, 1961. 



Morton, N. E.. 'The Mutational Load Due to Detrimental Genes in Man," Amer. J. 

 Human Genet., 12:348-364, 1960. 



Muller, H. J., "Mutational Prophylaxis," Bull. N.Y. Acad. Med., 2nd Ser., 24:447-469, 



1948. 

 Muller, H. L. "Radiation Damage to the Genetic Material," Amer. Scientist, 38:33-59, 



126, 399-425, 1950. 



Miintzing, A., "A Case of Preserved Heterozygosity in Rye in Spite of Long-Continued 

 Inbreeding," Hereditas, 50:377-413, 1963. 



