343 



two chromosomes are brought together in the female, each restores the 

 missing part to the other and a red-eyed female should result in the Fi 

 generation. As a matter of fact no red-eyed female appears. She is invariably 

 a compound, that is, in each case she is intermediate between the eye colors 

 of the two stocks used as parents. 



Again the evidence is fairly conclusive that when the two X chromosomes 

 are brought together in the female they break and reunite at apparently 

 all levels on the chromosome. Accordingly, it would seem that a break 

 and reunion would occasionally take place between the members of this chain 

 of loci. If such a phenomena should occur a complete chain of loci would 

 result like the chain in Fig. C (on the extreme left) , which would express itself 

 in the F 2 generation in the production of a red-eyed male. But in all the 

 possible attempts to break up such a line, as shown in Fig. D, no such a red- 

 eyed male has been found. To be sure the loci may be so close together 

 that crossing over would take place infrequently, but the evidence from such 

 large counts as have been made in which the red-eyed male has been 

 specifically looked for would weigh heavily against its ever taking place. 



3. The mutations may be due to losses according to the scheme repre- 

 sented in Fig. B., one loss produced blood, two losses produced cherry, 

 and so on. Such an assumption would seem to accord with the fact that when 

 any two of these stocks are crossed no red females are produced in the Fi 

 generation. On the other hand it should be expected that the chromosome 

 in which the least number of losses had occurred would act as a dominant. 

 For example, when blood and tinged are crossed, the females should be like 

 blood. But no such result is obtained. The female is intermediate in color. 



Again we should expect from the phenomena of crossing-over that, in 

 a cross for example between blood and white occasionally a cherry, or an eoein, 

 or a tinged male would appear in the F 2 generation, but none has been ob- 

 served. 



4. The history of the appearance of the members of this multiple allelo- 

 morph system shows that they are rare phenomena. Careful observation 

 by Morgan, Sturtevant, Muller, Bridges, myself and others show these 

 mutants to have appeared but a few times. It would be safe to say, I think, 

 that only one has occurred in five million times. I have represented blood 

 by one loss from the chromosome. Tinged is the result of four losses in 

 this completely linked chain of loci. The possibility of such mutants appear- 

 ing involves so many simultaneous losses that there would be one chance 



