220 



AN INTRODUCTION TO MODERN GENETICS 



and the same as the normal 2 A, 2X, whereas, of course, the differences 

 4^ — 4Ar, 3/4 — ^X and A — X would be different. (The haploid 

 material is only known as patches of tissue in mosaic flies.) Further, 



Fig. 102. Drosophila melanogaster. — Below are the wild-type female and the 

 wild-type male. Above on the left is a female type intersex, and on the right a 

 male type. Both are triploids with only two X chromosomes; there may be only 

 two /Vths, and a Y is present in the chromosome group on the left. 



(After Bridges.) 



the presence of Y chromosomes makes no difference to the sexuality 

 either of ordinary males and females or of intersexes ; although parts 

 of the Y are necessary for fertihty in males they have no effect on 

 sexual differentiation. 

 The sex of an individual is therefore determined by the ratio between 



