252 THE THEORY OF THE GENE 



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+2X=? 3a+2)X=9 4a+4X=2 



Za+ X+Y=d' Sa4-X4-Y= Super rf 4a+2X4-Y=d' 



3a+2X= Inters ex 

 5a+2X+Y= " 



Fig. 138. 

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

 intersexes of Drosophila melanogaster. The tetraploid male (4a+ 

 2X + 2Y) is hypothetical. Besides these classes the triploid female 

 gives rise to superfemales (2a+3X). (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, 2a4-2X, 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 

 expectation for the tetraploid male, 4a-l-2X+Y (that has 

 not yet been obtained), is that he will be like the normal 

 male, since the balance is the same in both. 



