igio] 



SHULL INHERITANCE OF SEX IN LYCHNIS 



117 



Lychnis, the difference between Bryonia and Lychnis is reduced to a 

 question of dominance of the hermaphrodite (or modified male) 

 condition. In order that Bryonia alba should breed true, the monoe- 

 cious (or modified male) character must be homozygous. The fact 

 that in Bryonia a cross between the female B. dioica and the monoe- 

 cious B. alba produces only females, indicates that the monoecious 

 character is capable of coming to full expression only when present 

 in the homozygous state. In other words, while the monoecists are 

 modified positive male homozygotes, absence of this modified male 

 character is almost perfectly dominant over its presence, so that the 

 heterozygous individuals which constitute the F x of this cross almost 

 or quite completely lack the male character. In this connection the 

 occurrence of occasional male flowers on a few of these females of 

 the F, may be of interest as showing a slight influence of the unpaired 

 gene for the modified male or monoecious sex. 



In Lychnis, on the other hand, the presence of the hermaphrodite 

 character is dominant over its absence, as demonstrated by the fact 

 that the hermaphrodite mutants are heterozygous with respect to 

 sex. In the following table a comparison may be made between the 

 two methods of explaining the character of the F t hybrid generation : 



TABLE III 

 ASSUMPTION: The female is a positive homozygote 



ASSUMPTION: The female is a negative homozygote 



5X3 

 ?X5 

 5X5 



$?55X??55 



?? and $93 



??55 



?and3 



$ 

 5 



2?X??5 



$?and??5 

 ??and??5 



$and5 

 ?and5 



No decision can be made between the two methods of explanation 

 contrasted in this table until the F 2 has shown the dependence or 



