INHERITANCE OF SEX AND RELATED PHENOMENA 213 



4. Bryonia alba self-pollinated gives only monoecious plants. 



If we assume that all the pollen grains and ovules of Bryonia alba 

 are of one kind which is indicated by the fact that it breeds true to the 

 monoecious condition, then there is no escape from the conclusion that 

 female plants of Bryonia dioica produce only one type of ovule but male 

 plants produce two types of pollen grains. Unfortunately as is often 

 the case in interspecific hybrids, the FI of this cross is sterile and con- 

 sequently the analysis cannot be carried further. 



Shull, however, has studied the inheritance of sex in Lychnis dioica 

 which is normally dioecious but occasionally produces hermaphroditic 

 plants. Although this case has not yet been fully analyzed, the results 

 thus far indicate clearly that the male is heterozygous with respect to a 

 sex-determiner, and the female homozygous. The results of Shull's 

 investigations may be stated under several definite heads as follows: 



1. Females with pollen from males give substantially equal numbers 

 of male and female offspring. 



2. Females with pollen from genetic hermaphrodites give equal 

 numbers of hermaphrodite and female offspring. 



3. Females with pollen from somatic hermaphrodites give equal 

 numbers of male and female offspring. 



4. Genetic hermaphrodites selfed give equal numbers of hermaph- 

 rodite and female offspring. 



5. Genetic hermaphrodites with males give equal numbers of male 

 and female offspring. 



6. Females from whatever source are genetically identical. Thus 

 females from the cross female X hermaphrodite transmit the same sex- 

 determiners as females from the cross female X male. 



7. In crosses between female and hermaphrodite a small percentage 

 of mutant males always appears and in crosses between female and 

 male approximately the same percentage of mutant hermaphrodites appears. 



In the above resume of the experimental evidence on sex-determina- 

 tion in Lychnis, the equality of sexes was only approximate, in fact females 

 usually occurred in excess, and sometimes in considerable excess. 



Shull has interpreted this evidence to indicate that in Lychnis the 

 hermaphroditic condition results from a modification of the male con- 

 dition, and that this modification is reversible as shown by the evidence 

 in 7, above. Interpreted in terms of the XY type of sex-inheritance then, 

 females are XX; males, XY; and hermaphrodites, XY'; and the change 

 from Y to Y' is reversible. Clearly the results indicate that males and 

 hermaphrodites are heterozygous with respect to the sex-determiner, 

 and females homozygous, although later investigations which have not 

 yet been fully interpreted indicate that some disturbing factors are at 

 work, at least in certain cases. 



