430 SCIENCE PROGRESS 



was invariably a Tall. Tallness, therefore, he called the dominant 

 character and dwarfness the recessive. 



These hybrids were allowed to self-fertilise (the normal 

 process in Pisnm), and seeds gathered from them gave, when 

 grown, 1064 plants, of which 787 were tall and 277 dwarf; that 

 is to say, the hybrid Tails produced Tails and Dwarfs in the 

 proportion of 2*84 to 1, or roughly 3 to 1. 



The seeds of these plants were sown, and it was found 

 that all the Dwarfs bred true, whilst of every three Tails, one 

 produced only Tails, the other two giving rise to Tails and 

 Dwarfs in the proportion 3 to 1. The foregoing account may 

 be summarised in the following diagram : 



(Pure) Tall (Pure) Dwarf 



1 (Pure) Tall 2 (Hybrid) Tails 1 (Pure) Dwarf 



Let us now look at the three kinds of Peas which compose 

 the generation in which Pure Tails, Hybrid Tails, and Pure 

 Dwarfs appear in the proportion 1:2:1. We already know 

 that all the Dwarfs breed true, that a third of the Tails breed 

 true, whilst the rest of the Tails produce Tails and Dwarfs 

 in the proportion 3 to 1. This proportion — 3 to 1 — is made 

 up in the same way as the same proportion in the preceding- 

 generation — namely, 1 Pure Tall ; 2 Hybrid Tails ; and 1 Pure 

 Dwarf. All these Dwarfs breed true ; all the Pure Tails breed 

 true ; and the Hybrid Tails produce Tails and Dwarfs in the 

 proportion 3 to 1. And so on. 



There are two sets of symbols which are very convenient 

 in describing these phenomena. 



The first set describes the different kinds of individuals ; 

 the second the generations. D denotes the dominant form (in 

 the case we have been discussing, the Tall), R the recessive 

 (the Dwarf), and D R the hybrid. 



P denotes the parental generation ; Fi, the result of crossing 

 the two parent forms — in our case, the Hybrid Tall ; F2 denotes 

 the next generation, i.e. that in which the dwarfs reappear in 

 the proportion 25 per cent. 



