12 CHAPTER 1 



SUMMARY AND CONCLUSIONS 



Organisms are assumed to contain an intrinsic genetic factor which is responsible for 

 like reproducing like. Ibis genetic factor is presumed to have a physical basis in genetic 

 material. 



The genetic material must be different in dilferent species of organisms, and may 

 be different in different lines or breeds of the same species. Variations in phenotype 

 m.i\ be due to genetic or environmental differences, or both. The contribution made 

 to phenotypic variability bj one of these two factors may be evaluated by holding the 

 other factor as constant as possible. 



Genotypic differences arise by the process of mutation. The genetic material is 

 presumably transmitted from parents to offspring by means of the cellular bridge be- 

 tween generations, and is assumed to be self-replicating and to arise only from pre- 

 existing genetic material. 



Studies of cell division in which nuclei divide mitotically reveal that, of all cellular 

 components, the chromosome is the structure most likely to serve as the genetic material 

 or as its carrier. This hypothesis receives support from several of the properties of 

 chromosomes which parallel established or assumed properties of genetic material. 

 Chromosomes come only from pre-existing chromosomes; different species have differ- 

 ent chromosomal compositions; the chromosome content is identical both quantitatively 

 and qualitatively in each cell of a line produced by asexual reproduction; each chromo- 

 some retains its individuality, mitotic cell generation after mitotic cell generation, re- 

 gardless of the nature of the other chromosomes present; chromosomes can occasionally 

 mutate, the mutant chromosome then replicating the mutant form. 



REFERENCES 



Darlington, C. D., and Janaki-Ammal, E. K., Chromosome Atlas of Cultivated Plants, 

 London: Allen and Unwin, 1945. 



Flemming. W., 1 879. "Contributions to the Knowledge of the Cell and its Life Phe- 

 nomena," as abridged and translated in Great Experiments in Biology, Gabriel, 

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

 240-245. 



Johannsen, W., 1909. Elemente der exakten Erblichkeitslehre. Jena. See also a trans- 

 lation of the summary and conclusions of his 1903 paper, "Heredity in Populations 

 and Pure Lines," in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood 

 Cliffs, N.J.: Prentice-Hall, 1959, pp. 20-26. 



Makino, S., An Atlas of Chromosome Numbers in Animals, Ames, Iowa: Iowa State 

 College Press, 1951. 



Mazia, D., "Mitosis and the Physiology of Cell Division," in The Cell, Vol. 3, Meiosis 

 and Mitosis, pp. 77-412, Brachet, J., and Mirsky, A. E. (Eds.), New York: Aca- 

 demic Press, 1961 . 



Ruddle, F. H., "Nuclear Bleb: A Stable Interphase Marker in Established Lines of 

 Cells in Vitro," J. Nat. Cancer Inst., 28:1247-1251, 1962. 



Schrader, F., Mitosis: the Movement of Chromosomes in Cell Division, New York: 

 Columbia University Press, 1953. 



Scientific American, Sept. 1961, Vol. 205, No. 3, "The Living Cell," articles by J. 

 Brachet and D. Mazia. 



