l8 SEX-LINKED INHERITANCE IN DROSOPHILA. 



white. In Colias eurydice the male is orange and the females are orange 

 or white. In Papilio turnus the male is yellow and the females either 

 yellow or black. Those cases are directly comparable to an eosin- 

 cherry population, except that in Lepidoptera the female is heterozy- 

 gous for the sex differential, in Diptera the male. 



Since in Drosophila the results are explicable on a sex-linked basis, 

 a similar explanation may apply to polymorphism in butterflies. By 

 suitable combinations of eosin and cherry most of the cases of poly- 

 morphism in butterflies may be simulated. To simulate the more 

 complex cases, such as that of Papilio polytes and memnon, another 

 allelomorph like eosin would have to be introduced. A population of 

 mixed cherry and white would give three somatic types of females 

 (cherry, cherry-white, and white) and two of males (cherry and white). 



FERTILITY AND STERILITY IN THE MUTANTS. 



Aside from the decrease in fertility that occurs in certain stocks 

 (a question that need not be treated here), there are among the types 

 described in the text two cases that call for special comment. When 

 the mutant type called "rudimentary" was first discovered, it was 

 found that the females were sterile but the males were fully fertile. 

 Later work has revealed the nature of the sterihty of the female. The 

 ovaries are present and in the young flies appear normal, but while in 

 the normal flies the eggs in the posterior portion enlarge rapidly during 

 the first few days after hatching, in the rudimentary females only a 

 very few (about 15) eggs enlarge. The other eggs in the ovary remain 

 at a lower stage of their development. Rarely the female lays a few 

 eggs; when she does so some of the eggs hatch, and if she has been 

 mated to a rudimentary male, the oflFspring are rudimentary females 

 and males. The rudimentary females mate in the normal time with 

 rudimentary or with normal males, and their sexual behavior is normal. 

 Their sterility is therefore due to the failure of the eggs to develop 

 properly. Whether in addition to this there is some incompatibility 

 between the sperm and the eggs of this type (as supposed to be the case 

 at one time) is not conclusively disproved, but is not probable from the 

 evidence now available. 



In the mutant called "fused" the females are sterile both with wild 

 males and with males from their own stock. An examination of the 

 ovaries of these females, made by Mr. C. McEwen, shows clearly that 

 there are fewer than the normal number of mature eggs, recalling the 

 case of rudimentary. 



It should be noticed that there is no apparent relation between the 

 sterility of these two types and the occurrence of the mutation in the 

 X chromosome, because other mutations in the X do not cause sterility, 

 and there is sterility in other mutant types that are due to factors in 

 other chromosomes. 



