18 MENDELIAN SEGREGATION 
half with the indifferent Y chromosome. Random 
meeting of eggs and sperm will give the result shown 
in the lower line of the diagram. There will bea3:1 
ratio, as in other Mendelian crosses, but the white 
individuals in F, will be males. The factor for red in 
the F, male will always stay in the X chromosome, so 
that all the female-producing spermatozoa will carry 
red, and consequently all F, females will be red. 
The males will have red eyes if they receive the red- 
bearing chromosome from their mother and white 
eyes if they receive the white-bearing chromosome 
from their mother. 
The reciprocal cross is made by mating a white- 
eyed female to a red-eyed male (Fig. 10). The 
daughters will have red eyes and the sons white eyes. 
If these are inbred their offspring will be red and 
white in equal numbers, and not the usual three 
reds to one white. The explanation of this new 
ratio is at once apparent as soon as the history of the 
sex chromosomes is studied. 
The two X chromosomes in the white-eyed mother 
carry the factor for white eyes. After ripening, each 
egg carries one white-bearing X chromosome. The 
single X chromosome of the female-producing sper- 
matozoon of the red-eyed father carries the factor for 
red eyes; the male-producing spermatozoa carry the Y 
chromosome which, as stated above, is indifferent. 
Any egg fertilized by a spermatozoon containing the 
red-bearing X chromosome will produce a red daugh- 
ter, because red dominates white. Conversely, any 
egg fertilized by the Y-bearing male-producing sper- 
