THE DETERMINATION OF SEX 



203 



determiners for both sexes and be heterozygous, as 

 indicated in Figure 58 ; and third, that the character 

 of sex follows the law of alternative dominance, ac- 

 cording to which in the male offspring the male 

 determiner dominates M(F), while in the female 

 the female dominates (M}F. 



This hypothesis is simply an attempt to explain 

 the numerical equality of the sexes, and also the fact 

 that the determiner for the opposite sex may be car- 



MALC 



GAMETES 



ZYGOTES 



M(F) M(M) (F)F F(M) 



MALE -DO NOT OCCUR FEMALE 



FIG. 58. Diagram to show Castle's 1903 theory of the heredity of sex. 



ried by either parent, but it leaves unanswered the 

 question of what causes "selective fertilization" 

 and "alternative dominance." 



There appears to be some evidence that selective 

 fertilization, which was assumed in Castle's 1903 

 theory, may actually occur under certain circum- 

 stances. For example, homozygous or pure yellow 

 mice, that is, mice with a duplex determiner for 

 yellow color, are not known. In breeding, all kinds 

 of yellow mice behave as if heterozygous or simplex 

 with respect to yellow color, for when any two yellow 

 mice are bred together, they produce a certain per- 

 centage of recessives which would not happen if they 



