22 MENDELISM CHAP. 



a gamete containing tallness with one bearing dwarfness. 

 Owing to the completely dominant nature of the tall 

 character, such a plant is in appearance indistinguishable 

 from the pure tall, but it differs markedly from it in the 

 nature of the gametes to which it gives rise. When the 

 formation of the gametes occurs, the elements represent- 

 ing dwarfness and tallness segregate from one another, so 

 that half of the gametes produced contain the one, and 

 half contain the other of these two elements. For on 

 hypothesis every gamete must be pure for one or other of 

 these two characters. And this is true for the ovules as 

 well as for the pollen grains. Such hybrid Y l plants, there- 

 fore, must produce a series of ovules consisting of those 

 bearing tallness and those bearing dwarfness, and must 

 produce them in equal numbers. And similarly for the 

 pollen grains. We may now calculate what should hap- 

 pen when such a series of pollen grains meets such a series 

 of ovules, i.e. the nature of the generation, that should 

 be produced when the hybrid is allowed to fertilise itself. 

 Let us suppose that there are 4 x ovules so that 2 x are 

 "tall" and 2 x are "dwarf." These are brought in con tact 

 with a mass of pollen grains of which half are "tall" and 

 half are "dwarf." It is obvious that a " tall" ovule has an 

 equal chance of being fertilised by a "tall" or a "dwarf" 

 pollen grain. Hence of our 2x "tall" ovules, x will be 

 fertilised by " tall " pollen grains and x will be fertilised by 

 " dwarf " pollen grains. The former must give rise to tall 



