12 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



III. GENETICS. 



Drosophila melanogaster has been more extensively used in the study 

 of genetics than any other organism, and the theory of heredity that 

 is now generally accepted is based chiefly on the results obtamed with 

 this fly. 



The first paper on the genetics of Drosophila was published in 1906 

 (Castle et al.) ; the first mutation in D. vielanogaster was reported in 

 1910 (Morgan). Since that time about 150 books and papers dealing 

 with heredity in the genus have been published. About 250 different 

 mutant types have been discovered and studied, and at least 10,000,000 

 living individuals have been etherized and examined by more than a 

 score of investigators. The problems studied include practically every 

 branch of the subject of genetics. Not only has Drosophila been the 

 most productive material for research in the subject, but it is now the 

 standard object for laboratory instruction, and is used as such in 

 many colleges and universities. 



The mutant-types produced b}^ D. melanogaster are of very many 

 kinds. Eye-colors ranging from pure white to deep sepia are known, 

 and general bodj'^-colors from pale yellow to dark black-and-brown. 

 The shape of the eye and the character of its surface are both affected 

 by mutation. The wings are shortened, changed in shape, have parts 

 lost, or are entirely wanting. The bristles may be deformed, increased 

 or decreased in number, or made small. The microchsetse may be 

 disarranged, reduced in number, or may occur in areas normally bare. 

 Extra veins may occur, or veins normally present maj^ be thickened, 

 weakened, displaced, reduced, or lost. The legs may be misshapen, 

 have missing parts, or be increased in number. Giants and dwarfs 

 are both known. There are tj^Des that always die before meta- 

 morphosis, and there are tj^Des in which the females are always sterile; 

 others in which the males are alwaj^s sterile. All these and many 

 others have been shown to differ from the usual or "wild-tjTpe" form 

 by definite, heritable, and relatively stable units known as genes. 



New or "mutant" genes arise only rarely, and their appearance is 

 not under control. Once arisen, they are perpetuated simply by 

 breeding from individuals that bear them. It has been found that the 

 250 or more genes known in D. melanogaster are not inherited entirely 

 independently, but fall into four groups. The members of any one 

 group are entirelj^ independent of all members of say other group, but 

 are more or less closely associated in inlieritance wdth the members 

 of their own group. These four groups correspond in size and in 

 many other ways with the four chromosome pairs of this species; 

 and it seems certain that the genes in any one group are associated 

 because they lie in the same chromosome pair, and for no other reason. 



