TABLE OF CONTENTS 



List of Illustrations xiii 



L The Fundamental Principles of Genetics 1 



Mendel's Two Laws 1 



Linkage 10 



Crossing-Over 14 

 The Simultaneous Interchange of Many Genes in 



Crossing-Over 20 



The Linear Order of the Genes 22 



The Theory of the Gene 25 



XL Particulate Theories of Heredity 26 



III. The Mechanism of Heredity 32 

 The Mechanism of Mendel's Two Laws 33 

 The Number of the Linkage Groups and the Number 



of the Chromosome Pairs 36 



The Integrity and Continuity of the Chromosomes 37 



Mechanism of Crossing-Over 39 



IV. Chromosomes and Genes 45 



V. The Origin of Mutant Characters 59 



VI. Are Mutant Recessive Genes Produced by 



Losses of Genes'? 72 



Recessive Characters and Absences of Genes 74 

 The Bearing of Reverse Mutation (Atavism) on the 



Interpretation of the Mutation Process 85 



The Evidence from Multiple Allelomorphs 92 



Conclusions 94 



VII. The Location of Genes in Eelated Species 95 



VIIL The Tetraploids, or Fourfold Type 105 

 Tetraploidy as a Means of Increasing the Number of 



Genes in a Species 130 



