THE GENETIC SYSTEM AS A WHOLE 171 



distribution of autosomes, and any two or more of the 

 group may be connected with the same autosome. 



Group III. Another large group of typical Mendelian 

 characters, 50 to 100 or more In number, following the 

 method of distribution of autosomes. Any two or more of 

 this group may be connected with the same autosome, but a 

 character of Group III is not linked with any character of 

 Group II. 



Group IV. A small group of typical Mendelian charac- 

 ters, only 5 or 6 known. These are not linked with any of 

 the characters of Groups I, II or III. 



All of the 300 or more characters known In this species 

 fall Into one or the other of these four groups. The nature 

 of the characters found In the different groups Is considered 

 on later pages. 



Thus here the number of linked groups Is the same as the 

 number of pairs of chromosomes. There are three large 

 groups and one very small one; likewise there are three 

 large pairs of chromosomes and one very small pair (figure 

 7). It Is natural to conclude therefore that the groups of 

 linked characters correspond to the pairs of chromosomes, 

 and that the characteristics In each group are connected with 

 some particular pair. As it turns out, there Is conclusive 

 evidence that this Is true. The evidence for each of the 

 groups may be summarized as follows : 



All the characters In Group I are sex-linked; that Is, they 

 follow exactly the distribution of the X-chromosomes. Any 

 character in this group follows a particular X-chromosome 

 and its descendants, wherever that chromosome goes. The 

 characteristics In Group I are thus certainly dependent on 

 genes In the X-chromosomes. 



The small group of characters known as Group IV has 

 been proved to be connected with the small chromosomes of 

 pair IV (see figure 7). This has been demonstrated in the 



