PHENOMENA OF INHERITANCE 



271 



they may appear in either sex though they are 

 usually transmitted from fathers to daughters, 

 or from mothers to sons ("criss-cross" inheri- 

 tance) in exactly the way in which the sex 

 chromosomes (X) are transmitted. Morgan 

 has therefore concluded that the factors for 



Flies 



Chromosomes 



XX X^ X W F* 



$ ? c? c? 



FIG. 59. Sex-linked inheritance of white and red eyes in 

 Drosophila. Parents, white-eyed male and red-eyed female; 

 Fj, red-eyed males and females ; F^ red-eyed females and equal 

 numbers of red-eyed and white-eyed males. The distribution 

 of sex chromosomes is shown to right of flies; X carries the 

 factor for red eyes, ^ tne factor for white eyes, stands for 

 absence of X- (After Morgan.) 



