GENETIC SYSTEM: RELATION TO CHARACTERISTICS 1 23 



similar to those of X and of the autosomes? Does it too 

 play a role in heredity? 



As we shall see, the Y-chromosome differs In many ways 

 from the other chromosomes. Let us first summarize the 

 main facts as to the occurrence and distribution of Y-chro- 

 mosomes. 



1. Some organisms have no Y-chromosome. Among these 

 are many Insects (see figure 3), the dog, the cat, the horse. 



2. In some organisms it Is small and appears to be de- 

 generate. This is the case in many Insects; also in man 

 (figure 19). 



3. In some organisms the Y-chromosome Is large, but 

 of a different form from Its mate X. Such Is the case In 

 Drosophila (figure 7). 



4. In some organisms Y Is not distinguishable from X 

 in form or size, but experiment shows that It differs from X 

 In function. 



5. The Y-chromosome descends from father to son only, 

 exclusively In the male line (see figures 24 and 25). Nor- 

 mally it never occurs In females. 



6. Hence any characters that depend on the presence 

 of a particular kind of Y will (a) be found in males only, 

 and will (b) never be Inherited through females (since Y 

 does not pass through females). 



The Y-Chromosome in Drosophila: The functions of the 

 Y-chromosome have been more thoroughly studied in Dro- 

 sophila than In any other organism. Most of the facts as to 

 this have already been brought out. In connection with our 

 account of the functions of the X-chromosome. We may 

 summarize as follows the known facts as to the functions 

 of the Y-chromosome in Drosophila : 



(a) The Y-chromosome, although it normally occurs in 

 males, is not required for the production of male Individ- 



