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PART ONE 



Formal Genetics 



The first task of genetics was to discover the rules of inheritance. The 

 young science, beginning its life at the time Mendel's papers were re- 

 discovered [1900], accomplished this task extremely quickly; within 

 twenty years, the chromosome theory had been placed on a firm basis 

 and the main outlines of the science of heredity were clear. This 

 development is dealt with in Part I. The first chapter is devoted to the 

 basic facts on which the chromosome theory rests, the next two to the 

 consequences of chromosome heredity at different stages of the chro- 

 mosome cycle and in abnormal chromosome cycles. The fourth chapter 

 describes peculiarities in the behaviour of whole chromosomes, while 

 the fifth deals with the parts of chromosomes. The study of the mechan- 

 ism of crossing-over, considered in Chapter 6, is probably the most 

 rapidly advancing part of formal genetics at the present time; it is one 

 of the most important lines of attack in the investigation of the nature 

 of the cell and its parts, and it links up with the study of the physico- 

 chemical nature of the gene to which we return in Part V 



