GENETIC SYSTEM: RELATION TO CHARACTERISTICS 9 1 



we may represent its cells as XY. Normally in forming the 

 germ cells, the two X's of the female separate into differ- 

 ent germ cells, and all the ova receive one of the modified 

 X's. The normal male produces germ cells of two classes, 

 X and Y. When the germ cell X from the female unites 

 with Y from the male, sons are produced, with the consti- 

 tution XY \ while X from the female with X from the 

 father gives daughters XA^. Since the yellow color de- 

 pendent on X is recessive, the daughters XX have the 

 normal gray color, like the father. But the sons A^Y have 

 but one X and as this is the defective one, from the mother, 

 the sons are yellow, like the mother. The normal result of 

 such a mating therefore is that all the sons are recessive 

 like the mother, all the daughters dominant, normal, like 

 the father. This is what occurs when the X-chromosomes 

 are distributed to the germ cells in the usual way. 



Abnormal Distribution (figure 26, B) : Sometimes, as 

 seen on previous pages, in forming germ cells both X's of 

 the female go together to a single ovum, while other ova 

 receive no X (see figure 20). In a race discovered by L. V. 

 Morgan ^ the two X-chromosomes in the female were 

 partly united, so that they almost invariably thus go to- 

 gether to one ovum, leaving other ova without an X. In 

 this race the X-chromosomes were so modified as to pro- 

 duce the recessive yellow color described in the preceding 

 paragraph. We therefore have an opportunity to determine 

 the course of inheritance of the yellow and the gray color 

 when the X-chromosomes are thus abnormally distributed. 



Represent as before the X-chromosomes that produce 

 the yellow color by X. Thus one set of ova receive XX, 

 while another set receive no X. (See the diagram, figure 

 26, B.) When the germ cells XX united with the germ cells 

 Y from the normal father, there were produced daughters 

 AAY, in which both A''s came from the mother, and these 

 daughters were yellow like the mother (since no normal X 



