50 SEX-DETERMINATION 



These results can be explained on the following assump- 

 tions: {a) 2X:2A=$XY:2A=(^, (b) in B. alba, which is a 

 homozygous strain, all individuals are females modified to 

 give staminate flowers by a male-determining mutation in an 

 autosome, {c) XX:A°iA°i=hermaphrodite. All the progeny 

 of the cross B. dioica ^ x B. alba (^ would be XX:AA°i and 

 are females with a tendency to produce male flowers. In the 

 reciprocal cross the females would be XXiAA^^ but the 

 males would be XY:AA°i and therefore 'stronger' than 

 B. dioica males. 



In maize, which is normally monoecious, a dozen and 

 more mutations have been found which modify the expres- 

 sion of sex (Emerson, 1924, 1932). By the use of certain of 

 these dioecious strains of maize have been produced. These 

 mutants provide strong support for the conception that the 

 sex-characters are the product of the action of many genes, 

 some male-determining and some female-determining, the 

 end result being decided by the interplay between these two 

 kinds. 



In Rumex acetosa Ono (1935) has described the occur- 

 rence of triploidy. The normal diploid female has 14 

 chromosomes, XX:6 pairs of autosomes. In the male there 

 are the same 6 pairs plus XYY. Individuals with the con- 

 stitution 2X+2Y+3A were found to be intersexes. In this 

 species the sex-determining genes on the X are pre- 

 dominantly female-determining, those on the autosomes 

 predominantly male-determining, and the Y is neutral. 



