The Role of the Chromosomes 213 



Usually the fruit fly has red eyes, but occasional individuals 

 occur with white eyes. If a red-eyed male be mated with a 

 white-eyed female the offspring will be of two classes of 

 approximately equal numbers, namely, red-eyed females and 

 white-eyed males. If now the first generation be inbred, four 

 classes will appear in the second generation, of approximately 

 equal numbers each, namely, white-eyed and red-eyed males 

 and females. If the reciprocal cross be made, i. e., white- 

 eyed males by red-eyed females, the results will differ depend- 

 ing upon the purity or impurity of the mother in respect to 

 eye color (i. e., whether or not she carries the white-eye color 

 latent). In the former case (the mother pure red) the first 

 generation will all have red eyes, while the second generation, 



Xx xx XY xY XX Xx XY xY 



DIAGRAMS KEPRESENTING THE E6LE OF THE CHROMOSOMES IN DETER- 

 MINING SEX AND EYE COLOR IN THE FRUIT FLY 



The female of this fly carries a pair of chromosomes, represented by 

 XX or xx in the diagrams, and the male a pair differing in respect 

 to one member, thus represented as XY or xY. The factor for red 

 eye color is represented as carried by X. Thus a red-eyed female has 

 the formula XX or Xx and a white-eyed female xx, while red-eyed and 

 white -eyed males are respectively XY and xY. In the right-hand 

 diagram is shown a mating between a white-eyed male and a red-eyed 

 female, all of the offspring of which are red-eyed. If these are mated 

 with each other, four kinds of offspring result, which are shown in 

 the third row, all the females having red eyes, while half the males 

 have white eyes. The reciprocal cross and its results are shown in the 

 left-hand diagram. 



resulting from inbreeding the former, will have red and 

 white eyes in the ratio of three to one. In the latter case (the 

 mother red-eyed with white latent) the results will be the 

 same as those obtained by inbreeding the first generation of 

 the red-eyed male by the white-eyed female cross. 



Sex in the fruit fly appears to be determined by a pair 

 of chromosomes, which differ slightly in form and size from 

 the others. This pair is called the XX pair in the female 

 and the XY pair in the male. 



