Chapter VII 



THE LINKAGE GROUPS AND THE 



CHROMOSOMES 



If the hereditary elements, the genes, are carried by 

 the chromosomes and if the chromosomes are per- 

 sisting structures, there should be as many groups 

 of hereditary characters as there are kinds of chro- 

 mosomes. In only a few cases has a sufficient number 

 of characters been studied to show whether there is 

 any correspondence between the number of heredi- 

 tary groups of characters and the number of chro- 

 mosomes. In the yinegar fly, Drosophila, there are 

 about four hundred characters that fall into four 

 groups. On page 88 {fig. 38) some of these are 

 giyen arranged according to groups. The charac- 

 ters are arranged in four grou2)s, Group I, II, III 

 and IV. Three of these grou23S are equally large or 

 nearly so; Group IV contains only three characters. 

 The characters are put into these grou2)s because, in 

 heredity, the members of each group tend to be in- 

 herited together, i.e., if two or more enter the cross 

 together they tend to remain together through sub- 

 sequent generations. On the other hand, any mem- 

 ber of one group is inherited entirely independently 

 of any member of the other groups ; in the same way 

 as Mendel's yellow-green pair of characters is in- 

 herited independently of the round-wrinkled pair. 



