THE GENETIC SYSTEM AS A WHOLE 



173 



pass to different offspring. They are evidently connected with 

 the two separated pieces of chromosome III. In the same 

 way the other large chromosome (II) may be broken, and 

 this causes the characteristics of the linkage Group II to 

 separate Into two groups. Minute study shows that the 

 chromosomes II and III differ a little, In size and form. 



N. 



ormal 



Eyeless 



.^-^ ^-^— -_ ^ 

 e" """"" """" ^^" 



N 



ormal 



Eye I 



yeiess 



Figure 40. Diagram of the mating that shows that the recessive 

 character eyeless (e) is dependent on a gene in chromosome IV. Of 

 the two parents, one, having but one chromosome IV, has normal 

 eyes; the other is eyeless. Forming germ cells and mating them in 

 the usual way, two types of offspring are produced in Fi. One type, 

 with two of the fourth chromosomes, is normal ; the other, with but 

 a single fourth chromosome, is eyeless, thus proving that the gene 

 for eyeless is in chromosome IV. For if e were in one of the other 

 chromosomes, the presence of a normal member of the pair would 

 prevent the manifestation of the recessive eyeless. 



so that with practice It becomes possible to distinguish one 

 from the other under the microscope. 



Thus It is proved that each of the four linkage groups 

 Is connected with one of the four chromosome pairs; the 

 four groups correspond to the four chromosomes. 



In a number of other organisms the number of linked 

 groups of characters has been determined, and also the 

 number of pairs of chromosomes. In every case In which 

 this has been fully worked out, the number of linked groups 



