216 



Biology in America 



tional evidence for the chromosome hypothesis as outlined 

 above. 



In the course of their experiments the students of the fruit 

 fly were suddenly confronted with an unexpected, and at 

 first sight unexplainable case, which seemingly set the chromo- 

 some hypothesis on its head. As already explained, if a pure 



'H.LOW. SPOT, 



wims. gosiN. i 



VCBMILION. 



I YIMATt HE. 



UII'DUOUM. 



25.0 PINK. PEA 



CHROMOSOME MAP SHOWING DISTRIBUTION OF LINKED CHARACTERS IN 



THE FRUIT FLY 



After Morgan, "Heredity and Sex." 

 By permission of the Columbia University Press. 



white-eyed female be mated to a red-eyed male, red-eyed 

 daughters and white-eyed sons in approximately equal num- 

 bers are the result. But in one set of experiments this cross 

 produced, in addition to the expected classes of offspring, 

 about 2.5% of white-eyed females and an equal number of 

 red-eyed males. Such a result is readily explainable how- 

 ever on the assumption that the two X chromosomes of the 



