172 GENETICS 



following manner (see figure 40). By non-disjunction of 

 this fourth pair of chromosomes, some individuals are pro- 

 duced having but one small chromosome (IV). That this is 

 the situation can be seen under the microscope, in properly 

 prepared material. 



These individuals with but one of the small chromosomes 

 have the dominant normal or "wild" characteristics. Such 

 individuals are mated with individuals containing one of 

 the recessive characters of Group IV (figure 40). One of 

 these recessive characters is "bent wings"; another is "eye- 

 less" — the individual having no eyes. 



Half of the germ cells from the individuals that have but 

 one of the fourth chromosomes are without this chromo- 

 some, while half of them have it. These two kinds each 

 unite with germ cells carrying the recessive character "bent," 

 or "eyeless." There are thus produced individuals, half of 

 which have but one of the fourth chromosomes, while the 

 other half have two (figure 40). In those which contain 

 two, all the characteristics are of the dominant normal 

 type; the recessive character bent or eyeless is not mani- 

 fested. But in the individuals having but one fourth chromo- 

 some, the recessive character is manifested: the individual 

 shows bent wings, or is eyeless. This demonstrates that the 

 characteristics of Group IV are connected with the small 

 chromosomes. 



Thus the large linkage Groups II and III are left for 

 the two large V-shaped chromosome pairs, II and III. These 

 chromosomes can be broken into two or more pieces by 

 subjecting them to radiations. When this is done, the char- 

 acters of the corresponding linkage groups are no longer 

 linked together. Thus in Group III the two genes scarlet 

 and sooty are linked (completely in the male). But when 

 the corresponding large chromosome (III) is broken into 

 two pieces, scarlet and sooty are no longer linked; they may 



