THE DETERMINATION OF SEX 



205 



TYPE 



I 



n 



ZYGOTES 



(Female) 9O* 

 (Male) Cfcf 



(Female^ 

 (Male) 



GAMETES 



$ + 

 rf 



d 1 



{Finale) (Male) 



99 Vcf 



bmfe) (hate 



(Hate) 



FIG. 69. Diagram to show the neo-Men- 

 delian theory of the heredity of sex, using 

 sex symbols. 



6. THE NEO-MENDELIAN THEORY OF SEX 



Correns (1906) avoids the difficulties of alternative 

 dominance, which Castle's hypothesis offers, by sup- 

 posing that one par- 

 ent only is hetero- 

 zygous with respect 

 to sex, and this sup- 

 position is becom- 

 ing more and more 

 probable as evidence 

 accumulates. Ac- 

 cording to this idea, 

 there are two types 

 of cases, one when the female is the heterozygous 

 parent and the other when the male is the hetero- 

 zygous parent, as represented in Figure 59. 



The formulae for these types may be expressed in 

 the nomenclature of the presence and absence theory, 



as follows (Fig. 60), 

 in which the sym- 

 bol x represents the 

 female determiner 

 in the heterozygous 

 case of type I, and 

 xx the female de- 

 terminer when the 

 male is the hetero- 

 zygous parent. 

 The formulae may be still further modified, accord- 

 ing to Morgan, for the satisfaction of those who ob- 



FIG. 60. Diagram to show the neo-Men- 

 delian theory of the heredity of sex accord- 

 ing to the presence and absence hypothesis. 



