16 MENDELIAN SEGREGATION 



fore supposed to be contained in them. These factors 

 are said to be sex linked. 



The inheritance of white eyes may serve as an 

 illustration for the entire group of sex linked char- 

 acters. If a white-eyed male is bred to a red-eyed 

 female (wild type) (Fig. 9), the sons and daughters 

 (Fi) have red eyes. If these are inbred the offspring 

 (F 2 ) are three reds to one white, but the white-eyed 

 flies are all males. If we trace the history of the sex 

 chromosomes we can see how this happens. 



In the red-eyed mother, each egg contains an X 

 chromosome bearing a factor for red eyes. In the 

 white-eyed father, half of the spermatozoa contain an 

 X chromosome which carries a factor for white eyes, 

 while the other half contain a Y chromosome which 

 carries no factors (Fig. 9) . Any egg fertilized by an 

 X-bearing spermatozoon of the white-eyed father will 

 produce a female that has one red-producing X chro- 

 mosome and one white-producing X chromosome 

 (Fig. 9). Her eyes are red, because red dominates 

 white. Any egg fertilized by a Y-bearing spermato- 

 zoon of the white-eyed father will produce a son 

 (Fig. 9) that has red eyes, because his X chromo- 

 some brings in the red factor from the mother, while 

 the Y chromosome does not bring in any dominant 

 factor. At the ripening of the germ cells in the Fi 

 female the number of chromosomes is reduced to 

 half. There result two kinds of eggs, half with the 

 red-bearing and half with the white-bearing X (Fig. 

 9). Similarly in the male there will be two classes 

 of sperm, half with the red-bearing X chromosome, 



