INHERITANCE OF SEX 335 



only, it is necessary to assume that the male plant is hetero- 

 zygous for the factor in question. A dioecious plant of this 

 nature really consists of two races living side by side, 

 differing in the possession by one of them of a factor 

 responsible for the suppression of femaleness and therefore 

 for the appearance of maleness. When the heterozygote 

 male breeds with the homozygote female (which is, of course, 

 the only possible mode of reproduction within the species) 

 equal numbers of the same two classes are always produced. 

 The case may be illustrated in the usual Mendelian 

 notation : — 



If Z be used to indicate the factor repressing herma- 

 phrodite flowers, then the two crosses with B. alba may be 

 represented : — 



B. alba = aazz ; B, dioica $ = aaZZ ; B. dioica ^ = aAZZ 



B. alba X B. dioica <^ 



aazz aAZZ 



/\ / \ 



/ \ , / \ 



az az ■^ aZ AZ 



i female = aazZ, aAzZ = J male 



B. dioica $ X B. alba 



aaZZ aazz 



/ \ / \ 



/ \ , / \ 



aZ aZ \I^ az az 



all female = aazZ 



The hybrids are unfortunately completely sterile and 

 further proof of the correctness of the theory by breeding 

 the second generation is not possible. Correns (1907) 



