CONTENTS xxiii 



PAGE 



The Seed 339 



Dormancy of Seeds 341 



The Chromosome History in Plants and Animals 342 



Reduction and Segregation of Chromosomes in Plants 342 



Vegetative Reproduction 346 



Section 9. Heredity 



CHAPTER XXVIII 



Mendel's Laws 



Variation , 349 



Pure Lines and Hybrids 349 



Modifications 349 



Mutations 350 



Recombinations 350 



Heredity 350 



Mendel's Discovery of the Laws of Heredity 350 



The Law of Segregation 351 



A Simple Illustration 351 



The Purity of Gametes 352 



The Recombination Ratio 352 



The Bearers of Heredity are Chromosomes 353 



The Chromosomes Contain the Genes 353 



Segregation of Chromosomes and Genes 354 



Dominance and the 3 : 1 Ratio 354 



The Law of Segregation 356 



The Relation of Genes to Characters 356 



Are Genes the Sole Bearers of Heredity? 357 



Are Acquired Characters Inherited? 357 



CHAPTER XXIX 



The Law of Independent Association 



The Independence of Characters 359 



The Combinations of Independent Characters in F « 359 



The Mechanism of Reassortment 360 



The Chromosomes of the Tall Purple Hybrid 360 



The Gametes of the Tall Purple Hybrid. 361 



The Combination of the Four Kinds of Gametes 361 



The 9:3:3:1 Ratio 362 



Genotypes and Phenotypes 362 



The Law of Independent Assortment of Genes and Characters 363 



CHAPTER XXX 



Linkage Phenomena 



The Third Law 364 



The Fourth Law 364 



The Vinegar Fly 365 



The Sex Chromosomes of Drosophila 366 



The Determination of Sex 366 



