INTERSEXES 241 



an ordinary female, it has one set more of the ordinary 

 chromosomes. It is clear from this that sex is determined 

 not by the actual number of the X-chromosomes present, 

 but by the ratio of these to the other chromosomes. 



Diploid Triploid Tetraploid 



2a-f2X=$ 3a+3X=$ 4a+4X=$ 



Za-r X+Y=d Sa+X4Y= Super d 4a+2X+Y=cf 



3a4-2X= Inters ex 

 2>a+2X+Y= 



3a(-IV)+2X= 



> > 



■>■> 



2>a(-IV)+2X= 

 +Y 



>3 



Fig. 138. 



Diagram giving the formulae of normal, triploid, tetraploid, and 



intersexes of Drosophila melanogaster. (After Bridges.) 



From these exceptional relations amongst the chromo- 

 somes, described by Bridges, he concluded that sex is 

 determined by a balance between the X's and the other 

 chromosomes. We may think of the X-chromosomes as 

 containing more of the genes that go to produce a female, 

 and the rest of the chromosomes as containing more of 

 the genes that go to produce a male. In the normal fe- 

 male, 2a+2X, the two X's turn the scale toward female- 

 ness. In the normal male there is only one X, and the 

 balance turns the other way. The triploid, 3a+3X, and 

 the tetraploid, 4a+4X, have the same balance as the 

 normal female and are practically identical with her. The 



