6 



What is a Gene? 



The complete answer to the question, what is a gene, involves more 

 detailed knowledge of nucleic acid chemistry and of the associations and 

 actions of genetic material in the cell than we possess at present. 

 A major share of the research efforts in genetics is nowadays directed 

 at this question and our knowledge of the gene is considerably improved 

 over what it was even a decade ago. But it must be appreciated at the 

 outset that the word "gene" does not refer to a substance in the same 

 way as does the word "water;" rather it refers to a concept as does the 

 word "molecule," or "atom, or "electron." Although hereditary mate- 

 rial consists of nucleic acid, a substance, the unit hereditary determinants 

 are identified only on the basis of their biological activity. Conse- 

 quently, these units can be defined only in an operational way; our con- 

 cept of them is only a logical inference from a vast array of observations. 



We have already pointed out how the hereditary unit which deter- 

 mines a specific function must be a polynucleotide in order to exist in 

 enough different states to provide the coded information for all the mani- 

 fold functions different genes perform. We also saw that even when 

 single nucleotides were changed within the functional unit a mutation 

 might result. As a consequence of such a change, the new functional unit 

 is mutant when compared as a whole with the wild-tvpe gene. How about 

 the unit of transmission? Is it the equivalent of the functional unit or of 

 the smallest subunit than can mutate? Or is it of some other dimension? 

 Throughout most of the development of genetics it was assumed that 

 the units of function, mutation, and transmission were the same indi- 

 visible structure. By considering new evidence which makes these con- 

 cepts sharper, we will investigate the relations between them, attempting 

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