MENDEL'S EXPLANATION 165 



pollen grain with a tall factor as by a pollen grain 

 with a dwarf factor ; that is to say, the following 

 unions will be, approximately, equally frequent : — 



Tall factor (borne x Tall factor (borne 

 by ovule) by pollen grain) 



Tall factor (borne x Dwarf factor (borne 

 by ovule) by pollen grain) 



Now let us consider the ovules with the dwarf 

 factors. These again have an equal chance of being 

 fertilised by a pollen grain with a tali factor as by 

 one with a dwarf factor ; that is, the following 

 unions will be, approximately, equally frequent : — 



Dwarf factor (borne x Tall factor (borne 

 by ovule) by pollen grain) 



Dwarf factor (borne x Dwarf factor (borne 

 by ovule) by pollen grain) 



All the ovules have now been accounted for, 

 and it will be seen that the result is four kinds of 

 unions of germ cells, when these unions are classi- 

 fied according to the factors contained in the two 

 germ cells uniting in each case. Moreover, these 

 four kinds of union are, on the average, equally 

 frequent. They may now be written in one list, 

 with their frequency written after them in per- 

 centage : — 



Tall-Tall , . . . 25 per cent. 



Tall-Dwarf .... 25 „ 



Dwarf-Tall . . . . 25 „ 



Dwarf-Dwarf . . • , 25 „ 



These are the kinds of unions of germ cells classified 



