CONTENTS 



CHAPTER PAGE 



I. Introduction * 15 



II. Mendel's First Law — Segregation of the Gexes 19 



III. The Mechanism of Segregation 39 



IV. Mendel's Second Law — The Independent Assortment of 



the Genes 59 



V. The Mechanism of Assortment 73 



VI. Linkage 80 



VII . Crossing Over 87 



VIII. Crossing Over and Chromosomes 96 



IX. The Order of the Genes 118 



X. Interference 126 



XI. Limitation of the Linkage Groups 133 



XII. Variation in Linkage 139 



XIII. Variation in the Number of the Chromosomes and its Re- 

 lation TO the Totality of the Genes 147 



XrV. Sex-Chromosomes and Sex-linked Inheritance 165 



XV. Parthenogenesis and Pure Lines 204 



XVI. The Embryological and Cytological Evidence that the 



Chromosomes are the Bearers of the. Hereditary Units . . 212 



XVII. Cytoplasmic Inheritance 219 



XVIII. Maternal Inheritance 227 



XIX. The Particulate Theory of Heredity and the Nature of 



the Gene 234 



XX. Mutation 247 



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