OPERATION OF THE GENETIC SYSTEM, AS 

 EXEMPLIFIED BY ITS RELATION TO SEX 



Chapters I and II present as it were the framework of 

 Genetics. Chapter I gave the nature of the evidence which 

 shows that the materials of heredity — the chromosomes — 

 affect the characteristics of organisms. Chapter II presented 

 a history of this material in the passage of generations. 

 But the central problems of genetics lie in the physiology 

 of this material. How does the genetic system operate in 

 affecting the characteristics of organisms? And what are 

 the phenomena of variation and inheritance to which it 

 gives rise? To these questions we now turn. 



Development and characteristics are affected in innu- 

 merable ways by the materials of heredity, as is shown by 

 the results of substituting one chromosome for another. 

 Among the most striking alterations so produced are 

 changes of sex. A study of the relations of chromosomes to 

 sex furnishes an excellent introduction to the physiology of 

 the genetic system: it indeed leads far into the subject. The 

 relations of chromosomes to sex will therefore be taken up 

 first, and treated with some fullness, as revealing the nature 

 of chromosomal action.^ 



In Chapter I It was set forth that in many organisms 

 the sex of the individual to be produced by an egg may be 

 changed by altering the chromosomes which the egg con- 

 tains. We wish to inquire how the chromosomes operate in 

 producing sex differences. 



55 



