vi PREFACE 



expected new findings have emerged which are forcing a revahiation and 

 broadening of concepts in the entire science. 



It has seemed timely, therefore, to attempt a summary statement, in the 

 form of a modern textbook, of what appear to us the essential features of 

 our science, and the principal directions of current inquiry. Such a state- 

 ment must be understood to be tentative, witii many areas of uncertainty, 

 as befits a science in a fluid state. When they seemed important, we have 

 included concepts about which there is considerable doubt, always indi- 

 cating where this is the case. After all, the testing of concepts, and not 

 their certaintv, is the heart of science. The basic tenets of genetics are 

 presented in terms of key illustrative experiments, and we have tried to 

 capture the critical mood of the scientist and the controversial aspects of 

 the findings as well as the facts. 



In this book we have abandoned the chronological approach and have 

 attempted a synthetic presentation, organizing the knowledge in a logi- 

 callv .sequential pattern wherever possible. Thus we begin not with 

 Mendel's pea crosses of 1866, but with the transformation experiment of 

 1943, which provided the first direct evidence of the chemical composition 

 of hereditarv material. In all fields of cell biology, the direction of prog- 

 ress has been towards a more mechanistic analysis of events on a physico- 

 chemical basis. In genetics, the largest stride in this direction occurred 

 with the chemical indentification of nucleic acids as carriers of hereditary 

 information. Nucleic acids may not be the sole hereditary determinants 

 in the cell, but at least they represent one class, the only class yet detected; 

 as such they provide the best frame of reference for a review of the facts 

 and fancies of genetics, as they appear in 1960. 



Although this is not a textbook of microbial genetics, experiments with 

 microorganisms are heavily cited because they have provided material for 

 many incisive experiments. We have limited ourselves to the discussion 

 of genetics at the cellular level, and have omitted the application of ge- 

 netic principles to scientific agriculture, to the experimental analysis of 

 evolution, and to related fields. Although in the past, cell genetics and 

 population genetics have been presented side by side, these fields have be- 

 come so divergent in methodology and frame of reference that it .seems 

 preferable to treat them independently. 



Genetics lends itself well to a formal presentation, largely abstracted 

 from descriptive details and, therefore, it can be understood by persons 

 who lack a background in biology. Until recently, even a lack of knowl- 

 edge of physics and chemistry was no barrier, but as new experiments ex- 

 plore the molecular behavior of hereditary determinants, an elementary 

 knowledge in the physical sciences becomes es.sential. 



This book is addressed to the curious-minded of all ages from college 



