l68 GENETICS 



some. All characteristics that depend on the same chromo- 

 some form what is called a linkage group. Since there are 

 different numbers of chromosomes in different organisms, 

 there are correspondingly different numbers also of linkage 

 groups. 



These linkage groups are of great importance in heredity. 

 They reveal to us many things about the nature and action 

 of the genetic system. We shall therefore examine these 

 groups in a typical organism, selecting for this purpose the 

 animal in which they are best known, namely, Drosophila 

 melanogaster. 



The Groups of Linked Characters in Drosophila 



The fruit-fly has, as we have seen, four pairs of chromo- 

 somes (figure 7). On the genes in these four pairs of 

 chromosomes depend several hundred known inherited 

 characteristics. These many characteristics form four linked 

 groups, according to their dependence on genes in the X- 

 chromosome, or in the chromosomes II, III or IV of figure 

 7. It is to be noted that all of the characteristics that depend 

 on genes in either one of a given pair of chromosomes con- 

 stitute a single linkage group, since all those characteristics 

 may become connected with the genes of a single one of the 

 chromosomes. Thus the number of linkage groups is the 

 same as the number of pairs of chromosomes. 



In the course of breeding successive generations, linkage 

 of characteristics shows itself in the fact that certain char- 

 acteristics (dependent on genes in the same chromosome), 

 which are present together in one parent, are present to- 

 gether also in the individual descendants. This is best seen 

 on comparing grandchildren with grandparents, as is illus- 

 trated in figure 32. In this case one grandparent had the 

 combination white-yellow, the other the combination red- 

 gray. The same two combinations reappear in the grand- 



