1 66 GENETICS 



Thus the normal, dominant, characters form a standard 

 condition, of which the defective and recessive mutated 

 characters are modifications. 



The diverse genes of any chromosome are designated, so 

 far as they have received names, by the name of one of the 

 modified characters which it helps to produce. Thus the gene 

 yellow, near the upper or left end of the X-chromosome of 

 Drosophila, produces in its mutated condition the modified 

 body color yellow; while in its unmodified condition it pro- 

 duces the normal gray color of the wild fly. In the maps of 

 figure 38, all the names apply to the modified or mutated 

 condition of the gene. For every such modified gene there is 

 a corresponding unmodified normal or wild-type gene. It is 

 the custom to designate the wild-type gene from which any 

 modified gene is derived by writing with the name or ab- 

 breviation for the modified gene the sign +. Thus yellow + 

 or y -(- or -}- y signifies the normal wild-type gene that 

 produces gray body color; white + or w + signifies the nor- 

 mal gene located at I, 1.5, which produces red eyes in place 

 of white ones. 



The genes of any one of the chromosomes, as X, or a 

 given autosome, become modified in many different ways, 

 so that different examples of the same chromosome have 

 different sets of genes. The diversity of genes in the chromo- 

 somes of different germ cells causes diversities in develop- 

 ment, with resulting different characteristics in the individ- 

 uals produced. The characteristics produced by the action 

 of the genes of any chromosome follow in the individuals of 

 successive generations the distribution to the offspring of 

 the genes that produce them. Thus many characteristics 

 follow the course of the X-chromosome ; many follow each 

 autosome; a few may follow the Y-chromosome. 



Since a large group of characteristics follows each 

 chromosome, the result is that characteristics pass from 

 parent to offspring in groups. As an X-chromosome passes 



