ORILLA STOTLER WERNER. 



and a preponderance of female tendency genes located in the 

 X-chromosomes. In the presence of one X the male-tendency 

 genes located in the autosomes overbalance the female tendency 

 genes in the single X-chromosome and a male results; on the 

 contrary the presence of two X-chromosomes overbalances the 

 male-tendency genes in the autosomes and the development is 

 thrown toward femaleness. 



Goldschmidt (1923) in his work on moth hybrids has come to 

 the conclusion that the female producing factors lie in the W- 

 chromosome. In crosses between certain races females are 

 gotten through non-disjunction in such a way that the W-chro- 

 mosome comes from the father and the Z-chromosome comes 

 from the mother, just the reverse of the usual method. In such 

 cases he finds that femaleness follows the W-chromosome. 



In the light of the results obtained by Bridges on sex-inter- 

 grades in Drosophila produced through triploidy or otherwise 

 unbalanced conditions of chromosomes, it is very clear that sex 

 is not controlled entirely by a particular chromosome either 

 singly or doubly represented as Wilson would say. There may 

 be genes in many of the autosomes as well as in the sex-chromo- 

 somes which individually tend to throw the balance toward 

 maleness or toward femaleness. But Bridges was dealing with 

 an X Y type of sex-determination and with male digamety. 

 His opposing tendencies are located in the X-chromosomes and 

 the autosomes. In addition to this his X-chromosomes carry 

 sex-linked characters. Bridge's scheme as may be seen, will 

 not work for the birds or moths where female digamety occurs 

 But if we adopt the suggestion of Goldschmidt (1923) that the 

 W-chromosome which passes from mother to daughter carries, 

 female-tendency genes only and in addition assume that male- 

 tendency genes are located in the Z-chromosomes of which two 

 occur in the male and one in the female, we have a mechanism 

 that in many cases would seem to take care of both sex-linked 

 characters and sex-determination. 



In the duck then, if the large unpaired element in the cells 

 of the female is one of a complex of three that go to make up the 

 sex-determining mechanism, the following scheme would be 

 entirely in accord with the parallelism of cytology and experi- 

 mental breeding. 



