xxiv CONTENTS 



PAQB 



Linkage in Vinegar Fly Crosses 366 



T\\e Hack Cross 366 



Bai-k Cross Involving Independent Characters 367 



Linked Cliaractcrs in the First Hybrid Generation 367 



Back-Crosi«ing Linked Characters 369 



The rfcgregjition of Linked Genes 370 



The Strength of Linkage 371 



Crossing-Cher 374 



The Mechanism of Crossing-Over 374 



The Fifth Uw of Heredity 374 



The Sixth Uw of Heretiity 374 



Sex-Linkage 375 



Red-Kyed Female X White-Eyed Male 376 



Red-Eyed Male X VVhite-Eyed Fenuilo 378 



Summary of Principles of Heredity 378 



CHAPTER XXXI 

 The Character Expression of Genes 



The Differential Genes or Factors 381 



The Relative Position of Genes 381 



The Mutations of Genes 383 



The (lametic Ratios of Hybrids 383 



The Numl)er of Genotypes of the Second Generation 385 



The Phcnotypes of the Second Generation 385 



Complementary Genes 386 



Supplementary Genes 386 



Inhibitory Genes 387 



Cumulative Genes 387 



Summary 389i 



CHAPTER XXXII 

 Plant Breeding 



The History of Plant Breeding 390 



The Economic Imimrtnnce of Plant Breeding 390 



The Problems of Plant Breeding 391 



The Mothods of Plant Breeding 392 



Pure Line Selection 392 



Improving Indian Corn 393 



Breeding Cotton 393 



S<>e(lless ( )ranges 394 



Disease ResiMance 394 



Frost Resistance 396 



Summary 395 



CHAPTER XXXIII 

 Animal Breedinq 



Reproduction in Higher Animals 397 



On Interbreeding 397 



