CONSTITUTION OF THE CHROMOSOMES 



Genes and Their Relations to Characteristics 



We have seen in preceding chapters that there are many 

 different modifications of a particular type of chromosome, 

 such as the X-chromosome, producing diverse character- 

 istics, which follow in their inheritance the distribution of 

 the chromosome that produces them. 



Do all of these diverse modifications affect the same 

 part of the chromosome; do they each affect the entire chro- 

 mosome? That is, does the chromosome act always as a 

 unit, a unit that is diversely modified in different cases, so 

 as to produce different results? Or is the chromosome made 

 up of different parts, with diverse functions: one part be- 

 ing modified when the eye color is changed, another part 

 when the form of the limbs or of the body is altered, and 

 so on? Are diverse characteristics influenced by different 

 parts of the same chromosome? 



Facts have been discovered which show that the chromo- 

 some is not a simple unit, but is composed of different parts, 

 with different functions. We shall examine some of the 

 more important of these facts and their consequences, tak- 

 ing the X-chromosome as a type. 



As will be recalled, a recessive character that results 

 from a modified or defective chromosome is not manifested 

 when a normal chromosome is present in addition to the 

 modified one. If the cells of an individual of Drosophila 

 contain an X-chromosome that is defective in such a way as 



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