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. 



When these red-eyed daughters and white-eyed 

 sons are inbred the possible combinations are shown 

 in the lower line of the diagram (Fig. 10). 



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 Mendehan ratio of 3 : 1 obtains, and 

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



The Inheritance of Two 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. 



