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