20 MENDELIAN SEGREGATION 



matozoon will produce a white-eyed son, because the 

 only X chromosome that the son contains is derived 

 from his mother, both of whose X chromosomes carry 

 a white-producing factor. 



^lien these red-eyed daughters and white-eyed 

 sons are inbred the possible combinations are shown 

 in the lower line of the diagram (Fig. 1 0) . 



There will be two kinds of eggs, one containing a red- 

 bearing, the other a white-bearing, X chromosome. 

 The female-producing spermatozoa will contain a 

 white-bearing X chromosome; the male-producing 

 spermatozoa will contain a Y chromosome. A red- 

 bearing egg fertilized by a female-producing sper- 

 matozoon will produce a red-eyed female; a white- 

 bearing egg fertilized by a female-producing spermato- 

 zoon will produce a white-eyed female. A red-bear- 

 ing egg fertilized by a male-producing spermatozoon 

 will produce a red-eyed male; a white-bearing egg 

 fertilized by a male-producing spermatozoon will 

 produce a white-eyed male. The resulting ratio is 

 1 red to 1 white, in both sexes. 



The distribution of the chromosomes explains how 

 in one cross the Mendelian ratio of 3 : 1 obtains, and 

 also how in the reciprocal cross there is a 1 : 1 ratio. 



The Inheritance of Tw^o or More Independent 



Pairs of Factors 



The application of the chromosome hypothesis 

 to crosses between races that differ in two pairs of 

 factors is illustrated by the following example (Fig. 



