PHENOMENA OF INHERITANCE 373 



tions have arisen in which the eye is white. 

 Such a mutation always appears in males, 

 though it may later be transferred to females, 

 as we shall see. If now a white-eyed male 

 and a red-eyed female are crossed all the Fi s 

 are red-eyed, but if these Fi s are interbred all 

 the females of Fa have red eyes while half of 

 the males have red eyes and the other half 

 have white eyes (Fig. 59) . On the other hand 

 if one of the Fi females of this cross is bred 

 with a white-eyed male half of the females of 

 F2 are red-eyed and half are white-eyed, and 

 half of the males are red-eyed and half are 

 white-eyed. 



If now one of these white-eyed females is 

 bred with a red-eyed male all the females of 

 the Fi generation are red-eyed and all the 

 males white-eyed ("criss-cross" inheritance) 

 and if these are interbred there are produced 

 in the F2 generation equal numbers of red- 

 eyed and white-eyed males and females 

 (Fig. 60). 



The distribution of the maternal and pa- 

 ternal sex chromosomes (X) exactly parallels 

 this distribution of this sex-linked character, 



