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 be a 3 : 1 

 ratio, as in other Mendelian crosses, but the white 

 individuals in F2 will be males. The factor for red in 

 the Fi male will always stay in the X chromosome, so 

 that all the female-producing spermatozoa will carry 

 red, and consequently all F2 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 b}^ the Y-bearing male-producing sper- 



