MENDELISM 



431 



We may now summarise the Mendelian phenomenon in 

 the following diagram : 



D 



R 



Fi 



F2 



F3 



F4 



DR 



What is the cause of this beautifully regular and definite 

 phenomenon ? Mendel answered the question in this ingenious 

 way. He believed that the germ-cell of the hybrid pea did 

 not represent a mixture of tallness and dwarfness, but that a 

 germ-cell represented either a tall or a dwarf, and, moreover, 

 that half of the germ-cells represented tall and half of them 

 dwarfs. Let us see what is the result of the union of — say — 

 a hundred male germ-cells, or pollen-cells, with a hundred 

 female germ-cells, or egg-cells. Before we proceed, let us make 

 certain of two points which are essential to the argument. 

 Mendel believed that a pure-breeding tall pea contained only 

 tall-representing germ-cells — in fact, that the reason that it 

 bred true was that it did contain them; and that the dwarf 

 contained only dwarf-representing germ-cells. Mendel had 

 also found that the result of a union between a " tall " germ- 

 cell and a " dwarf" one was a hybrid which bore the tall 

 character. Now let us return to our argument. 



Fifty of the hundred pollen-cells would be " tall," and fifty 

 "dwarf." Let us consider the "tails" first. As half of the 

 eggs-cells are "tall" and half "dwarf," the "tall" pollen-cell 

 has an equal chance of meeting a " tall " and a " dwarf " egg- 

 cell. So that twenty-five of the " tall " pollen-cells would meet 

 "tall" egg-cells, and twenty-five would meet "dwarf" ones; 

 and the resulting unions would be (writing T for Tall, and 

 D for Dwarf): 



25 TT 

 25 TD 



