THE GENETIC SYSTEM AS A WHOLE 165 



The cytoplasm might correspondingly be called the "mate- 

 rials of bodily differentiation." 



Following the prevailing usage therefore we may define 

 the genetic system or material of heredity as follows: 



The genetic system consists of those parts of the cells 

 that are effectively diverse in different germ cells, these 

 diversities causing different characteristics to appear In the 

 different individuals produced. 



In rare cases, in certain plants, it Is found that the cyto- 

 plasm contains color bodies, which may differ in different 

 germ cells, and so cause differences In the characteristics of 

 the individuals developed from them. In such cases the 

 cytoplasm, or at least the color bodies it contains, must be 

 Included as part of the genetic system.^ But in most organ- 

 isms the genetic system consists of the chromosomes with 

 their genes. 



Constitution of the Chromosomes. 

 The Groups of Characters. Linkage 



Each of the chromosomes, (save In some cases the Y- 

 chromosome) is composed, as we have seen, of many diverse 

 genes, as represented In the maps of figure 38. In the wild, 

 unmodified Individuals, In the fruit-fly, the genes are such 

 as produce the "normal" or "wild-type" characteristics of 

 the individuals. The genes of any chromosome become mod- 

 ified In the course of time. In the process known as mutation, 

 which will be dealt with in a later chapter. The modified 

 genes produce altered characteristics, most of which are 

 recessive, while the normal characteristics, when mated to 

 the modified characteristics, are dominant. A few of the 

 modified characteristics however are dominant when mated 

 with the normal or wild-type characters. The modified 

 characteristics resulting from mutation are in most, if not 

 all, cases defects, as compared with the normal characters. 



