CYTOLOGY AND MENDELIAN HEREDITY 



187 



junction in the male, however, yields gametes of two classes: half the 

 sperms carry A' and half carry )'. The autosomes are the same in all 

 gametes, excepting of course any differences due to ordinary hetero- 

 zygosity in the individuals producing them. Syngamic unions of two 

 kinds are now possible, A' with X and A' ^nth Y. These yield, respec- 

 tively, offspring of two sexes: females (A" A') and males (AF). This 

 type of mechanism is found in man (Fig. 133), although the large number 



Animal 

 XY 



mm 



Female 



Animal 



XX 



M«iosi9 





Mai* 



Animal 



XY 



Egg and 

 Polocytej 



Female 



Animal 



XX 



Slaminaf e 

 Plant 

 XY 





Male 



GameJophyte 



Y 



Pistillate 



Plant 



XX 



Male 



Gametophyte 



X 



Female 



Gametoptiyte 



X 



Megosporei 



Staminate 



Plant 



XY 



Pistillate 



Plant 



XX 



Sporophyte 

 XY 



Spores 



Male 



Gametophyte 



Y 



Female 



Gametophyte 



X 



Sporophyte 

 XY 



Fig. 132. — Diagram illustrating the relation of the -Y- and F-chroraosonies to sex in the 

 life cycles of an animal, an angiosperm, and a bryophyte. 



of chromosome pairs (24) and their small size have made it difficult 

 to identify the sex pair, particularly the F. 



Many dioecious angiosperms have the same mechanism (Figs. 134, 

 135). Here the pistillate plant has AA and the staminate plant A}'. 

 All megaspores, female gametophytes, and eggs are alike in carrying A. 

 Half the microspores, pollen grains, and male gametes carry A', while the 

 other half carry F. Here again two sorts of combination are possible: 

 A with A, giving pistillate offspring, and A' with F, giving staminate 

 offspring. 



An example of the operation of the AF mechanism in monoploid 

 organisms is afforded by the liverwort Sphaerocarpos, the first plant 



