INHERITANCE OF SEX AND RELATED PHENOMENA 215 



birds, and various fantastic, ornamental, and combative characters, 

 usually confined to the male. Much historical interest attaches to 

 secondary sexual characters because of the attention directed to them 

 by Darwin's theory of sexual selection. With that we have no particular 

 concern in the present chapter, but shall only consider the inheritance of 

 them in one form as it is related to the inheritance of sex. 



In the foregoing discussion no particular reference has been made to 

 sex-factors, because after all so little is known concerning them. In 

 some cases we have found sex accompanied by differences in chromosome 

 content, one sex containing an equal pair of chromosomes which are 

 represented in the opposite sex by an unequal pair, in another case the 

 difference in sex appears to depend upon whether the individual possesses 

 the haploid or diploid number of chromosomes. We have also noted 

 that there are two different types of sex-inheritance, one in which the 

 male is heterozygous and the other in which the female is heterozygous. 

 It is only fair to conclude, therefore, that until more light is thrown 

 upon these matters, the assumption that sex-determination depends 

 upon a sex-factor rests on a rather slender basis. The experimental 

 evidence, it is true, is strictly analogous to certain types of Mendelian 

 inheritance, and an interpretation of the sex-factor may be given which 

 does no violence to our ideas of the complexity of sex-differences. Thus 

 it has been shown by ample evidence that the color of eyes in Drosophila 

 depends upon the cooperation of a number of different factors; we can- 

 not say definitely how many, but mutational changes have indicated 

 that over twenty-five different loci have something to do with the re- 

 actions concerned in pigment production in the eye. Yet in spite of 

 this fact the presence of a single factor may make all the difference 

 between a red eye and a white eye. Similarly the sex-factor may act 

 in conjunction with a whole series of other factors, yet the difference 

 dependent upon its presence in the homozygous or heterozygous con- 

 dition may make all the difference between the two sexes. At least in 

 one form, however, we have even more definite evidence of the presence 

 of a definite sex-factor. Shu 11 has shown in Lychnis that where males 

 are expected, hermaphrodite mutants occasionally appear. If we offer 

 the same explanation for the occurrence of these mutants as we have 

 offered for the occurrence of mutations in Drosophila, a change in a 

 single locus in the hereditary system, then the appearance of these 

 hermaphrodites might be offered as almost conclusive evidence of 

 the sex-determining action of a single sex-factor in this particular case. 

 The evidence here becomes even stronger when we consider the fact that 

 this particular type of change is reversible. Some additional light may 

 be thrown upon this question by the consideration of secondary sexual 

 characters as related to the inheritance of sex, although thus far the 



