440 Genetics of Sex Determination 



TABLE 6 

 (From M. Westergaard, 1948) 



the offspring of the triploids if female-determining autosomes occur as 

 aaaa and so on, and male ones only as aa. 



Westergaard thus draws the final conclusions: Melandrium is 

 endowed with two different mechanisms of sex determination, a pri- 

 mary sex-producing mechanism and a secondary sex-deciding one. 

 Female and male primary genes control the development of female 

 and male sex organs. These genes are located in all autosomes, some 

 of which contain more male genes and others more female genes. 

 If this mechanism were present alone, it would produce a hermaphro- 

 dite. In the dioecious organism this primary sex-producing mechanism 

 interacts with a secondary sex-deciding mechanism which permits 

 only one group of genes to function in one individual. The secondary 

 mechanism is located in the Y-chromosome. The X-chromosome of 

 Melandrium has indeed female determiners, but these are only some 

 of the primary sex genes and have nothing to do with sex decision. In 

 addition, the female-inhibiting function of the Y-chromosome may be 

 prevented by an accumulation of female-determining X-chromosomes 

 and autosomes, while the male action of the Y cannot be replaced by 

 an accumulation of male-determining autosomes or diminution of the 

 number of X-chromosomes. (Parenthetically I may mention that Hart- 

 mann tried to explain these facts by the obsolete formula of Correns, 

 FF = 9 , MF = $ , which fails here as everywhere else to explain the 

 details.) 



Wcstergaard's interpretation is not only cumbersome and full of 

 ad hoc assumptions but also has the disadvantage of being developed 

 only for the interpretation of a single aberrant case, and is com- 



