20 MENDELISM CHAP. 



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 F I plants, therefore, 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 happen when such 

 a series of pollen grains meets such a series of ovules, 

 i.e. the nature of the generation that should be pro- 

 duced when the hybrid is allowed to fertilise itself. 

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

 are " tall " and 2.x are " dwarf." These are brought 

 in contact 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 2.x " 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 plants, and since the dwarf character has 

 been entirely eliminated from them they must in 

 the future breed true. The latter must also give 

 rise to tall plants, but since they carry also the 

 recessive dwarf character they must when bred from 

 produce both tails and dwarfs. Each of the 2x 

 dwarf ovules, again, has an equal chance of being 

 fertilised by a "tall" or by a "dwarf" pollen grain. 

 Hence x will give rise to tall plants carrying the 

 recessive dwarf character, while x will produce 



