Contents 



1. The chemical basis of heredity 1 



Identification of Hereditary Material 3 

 Genes and Gene Action 28 



2. The mutable unit of heredity 34 



The Particulate Nature of the Genetic Unit 35 



The Reversibility of Mutation 39 



An Operational Definition of the Mutable Unit 40 



The Randomness of Natural Mutation 43 



Mutation Rates 53 



Conclusion 57 



3. Recombination in sexual organisms 60 



Segregation and Assortment 60 



Linkage, Recombination, and Genetic Maps 64 



Genes on Chromosomes 66 



Crossing over and Chromosome Mapping 73 



Limitations in Mapping 78 



Transmission Genetics of Diploids 83 



4. Cytogenetic correlations and crossing over 91 



Chromosomes as Seen in the Light Microscope 93 

 Chromosome Rreakage and Rearrangements 96 

 Variations in Chromosome Number 104 

 C ross in g Over 1 06 



5. Recombination in \iruses and bacteria 113 

 Mating among Viruses 114 



