14 



CHAPTER 2 



But you should recognize that the properties 

 of these genes must be expressed in terms of 

 recombination, the operation by which they 

 were detected. It is conceivable that other 

 operations will also partition the genetic 

 material into units, whose characteristics will 

 have to be expressed in terms of the opera- 

 tions employed for their detection. However, 

 until it is found that the different units re- 

 vealed by different operations are not equiva- 

 lent, we shall use the term gene to refer to any 

 unit of the genetic material, regardless of the 

 operation by which the unit was discovered. 

 In most of the genetic hterature to be re- 

 ferred to subsequently, as well as in the 

 greater portion of this book, it is usually 

 simple for the reader to determine from the 

 context of the discussion which operations 



are involved when the term gene is used. 

 The possibility exists that the genetic ma- 

 terial may be shown, via the study of genetic 

 recombination, to contain more than two 

 genes, in which case the maximum size of a 

 gene would be reduced. Henceforth, we 

 will be interested in any attempts to learn to 

 what extent the genetic material can be par- 

 titioned into genes, for such work leads us to 

 a better understanding of the nature of the 

 genetic material in terms of its recombina- 

 tional properties. Remember that near the 

 beginning of this Chapter we chose to inves- 

 tigate the genetic material from its recombina- 

 tional properties, and have postponed the 

 study of the nature and consequence of the 

 genetic material as revealed by other, non- 

 recombinational, techniques or operations. 



SUMMARY AND CONCLUSIONS 



Genetic material is assumed to be self-replicating and to arise only from pre-existing genetic 

 material. The term gene is used to refer to a unit or restricted portion of the total genetic ma- 

 terial as discovered via any operational procedure. The genes discovered in the present 

 Chapter were revealed by recombination. 



Genes occur in pairs. When they are transmitted in sexual reproduction the members 

 of a pair segregate so that any offspring receives only one member of a pair from each parent. 

 The gene is uncontaminated by the type of gene that is its allele prior to segregation, and 

 enters the new individual uninfluenced by the type of gene being contributed from the 

 other parent. 



REFERENCES 



Mendel, G., 1866. "Experiments in Plant Hybridization," translated in Sinnott, E. W., 

 L. C. Dunn, and Th. Dobzhansky, Principles of Genetics, 5th Ed., New York, 

 McGraw-Hill, 1958, pp. 419-443; also in Dodson, E. O., Genetics, the Modern Science 

 of Heredity, Philadelphia, Saunders, 1956, pp. 285 311; also in Classic Papers in 

 Genetics, Peters, J. A. (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 1959, pp. 1-20. 



The Birth of Genetics, Supplement to Genetics, 35, No. 5, Part 2, 1950, 47 pp. Contains 

 English translation of G. Mendel's letters to C. Nageli (1866-1873), and papers by 

 H. De Vries, by C. Correns, and by E. Tschermak published in 1900. 



QUESTIONS FOR DISCUSSION 



2.1. How would you recognize that a line of garden peas had become genotypically pure 

 for a given trait? 



2.2. Criticize the assumption that genes come only from pre-existing genes and do not 

 arise de novo. 



