CHAPTER 1 



eral, many times, or continuously? We may 

 get some answer by considering certain or- 

 ganisms, composed of but a single cell, which 

 reproduce asexually by dividing into two 

 cells. In this process the parent becomes ex- 

 tinct, so to speak, its individuality being re- 

 placed by two daughter cells of the same 

 kind. Once formed, the two daughters often 

 separate, never to meet again. In such a case, 

 at least, the genetic material, whatever it is, 

 must have been transmitted before the com- 

 pletion of cell division. .Accordingly, we should 

 examine this process of cell division in some 



detail for clues concerning the physical basis 

 of the genetic factor. 



The preceding reasoning has led us to 

 postulate the existence of genetic material, 

 which is transmissible and mutable (capable 

 of mutation), and which together with the en- 

 vironment determines phenotypes. But be- 

 fore we examine cell division for additional 

 evidence of some physical basis for the genetic 

 material, let us consider some work which 

 may provide us with more information with 

 regard to the transmissive properties of the 

 genetic material. 



SUMMARY AND CONCLUSIONS 



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

 reproducing like. This genetic factor is presumed to have a material basis. 



Accordingly, the genetic material must be different in different species of organisms and 

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

 may be due to either or both genetic and environmental differences. The contribution of 

 one of these two factors to phenotypic variability may be detected by avoiding variability 

 in the other of these two factors. 



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

 transmitted from parents to offspring by means of the cellular bridge between generations. 



REFERENCES 



Johannsen, W., 1909. Elemente der 

 exakten Erblichkeitslehre. Jena. See 

 also a translation of the summary and 

 conclusions of his 1903 paper, "Hered- 

 ity in Populations and Pure Lines," in 

 Classic Papers in Genetics, Peters, J. A. 

 (Ed.), Englewood Cliffs, N.J., Prentice- 

 Hall, 1959, pp. 20-26. 



WiLHELM LUDWIG JOHANNSEN 



(1861-1926). (By permission of 

 Genetics, Inc., vol. 8, p. 1, 1923.) 



