MENDEL S EXPERIMENTS 



68 1 



and shortness, which had disappeared in what is now called the 

 first filial, or F^, generation, he called recessive. The next thing 

 was to breed from the F^ generation by allowing the plants to 

 self-pollinate. In the second filial generation, or Fg, Mendel 

 grew 1,064 plants, and of these 787 were tall, and 277 dwarf ; 

 the recessive character had thus reappeared in approximately 

 one-quarter of the grandchildren of the original parents. In the 

 F3 generation the dwarfs from the Fg gave nothing but dwarfs, 

 while of 100 F2 tails 28 gave only tails, and 72 gave both tails 

 and shorts in the approximate ratio of 3 : i. The crosses involving 

 the other characters gave similar results and similar ratios, 

 the larger the number of results the closer being the ratio to 

 3:1. Mendel was thus able to state as a rule for the pea crosses 

 that when the hybrids form seeds they do so in the proportion 

 of one which is a pure dominant to one pure recessive to two 

 hybrids. He carried some of his experiments through seven genera- 

 tions from the original parents, getting similar results each time, 

 from which he deduced that the hybrids would never become pure. 

 The pure dominants and recessives are now called collectively 

 homozygotes, and what Mendel termed hybrids are now known 

 as heterozygotes. His experiments may be summarised in the 

 form of a genealogical table ; capital letters stand for the dominant 

 character, lower-case for the recessive, and the two together for 

 the hybrid. For reasons which appear later, it is better to represent 

 each homozygote by a double letter. 



AA 



aa 



Fi 



Aa 



AA 



1 

 4 



AA 



AA 

 i 



Aa 



2 



Aa 



and so on 



aa 



1 

 4 



aa 



i 



aa 



The next step that Mendel took in the analysis was to find 

 out what happened when the parents differed in two or three pairs 



