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THE AMERICAN NATURALIST [Vol. XL V 



form (the brownish tailed Achates) as the original type, 

 from which the tailless dark-colored male and the some- 

 what similar Laocoon have been derived by mutation, 

 in the same way that the white color, dominant in the 

 female but recessive in the male of Colias, may be pos- 

 tulated as the ancestral color in that genus. 



5. Inheritance of Sex 

 Discussion of the inheritance of sex in Colias philodice 

 at present must deal in part with unverified hypotheses, 

 because I have not j r et secured and tested white males 

 nor, if they exist, homozygous white females. Since, 

 however, all other possible combinations have been rea- 

 lized, these may now be reviewed, and tentative predic- 

 tions made as to what progeny may be expected in the 

 future from homozygous white stock in its various com- 

 binations. 



Let us suppose that the male color pattern and all 

 primary and secondary sexual characters of the male are 

 dependent upon the presence of a " determiner " for 

 which the male individual is a homozygous dominant 

 (xx), while the female individual is heterozygous, one 

 half of the gametes which it produces containing the 

 determiner (x) and one half lacking it (o). Thus the 

 gametic constitution of the female may be represented as 

 xo, that of the male as xx. 



Taking color into consideration, the nature of the pure 

 yellow male may be represented by the symbol : yyxx, 

 that of the pure yellow female as yyox. Furthermore, 

 if yellow is dominant in the male, and white in the female 

 the male heterozygote would be y(w) xx, while the white 

 female would have the symbol w(y) ox. Such a white 

 female, being heterozygous in both color and sex, may be 

 further assumed to produce in equal numbers gametes 

 of four kinds. This hypothesis will appear perhaps 

 more firmly grounded if we imagine that both of the 



ferential divisions, instead of one being an equation 

 division and one a differential division, as is usually as- 



