20 MENDELISM chap. 



the former to one of the latter. Counting showed that 

 the number of the tails which gave dwarfs was double 

 that of the tails which bred true. 



If we denote a dwarf plant as D, a true breeding tall 

 plant as T, and a tall which gives both tails and dwarfs 

 in the ratio 3 : i as T (D), the result of these experiments 

 may be briefly summarised in the foregoing scheme.^ 



Mendel experimented with other pairs of contrasted 

 characters and found that in every instance they followed 

 the same scheme of inheritance. Thus coloured flowers 

 were dominant to white, in the ripe seeds yellow was dom- 

 inant to green, and round shape was dominant to wrin- 

 kled, and so on. In every case where the inheritance of an 

 alternative pair of characters was concerned the effect of 

 the cross in successive generations was to produce three 

 and only three different sorts of individuals, viz. domi- 

 nants which bred true, dominants which gave both 

 dominant and recessive offspring in the ratio 3:1, and 

 recessives which always bred true. Having determined a 

 general scheme of inheritance which experiment showed to 

 hold good for each of the seven pairs of alternative char- 

 acters with which he worked, Mendel set himself to pro- 

 viding a theoretical interpretation of this scheme which, 

 as he clearly realised, must be in terms of germ cells. He 



' It has been found convenient to denote the various generations 

 resulting from a cross by the signs Fi, F2, F3, etc. Fi on this system 

 denotes the first filial generation, F2 the second filial generation pro- 

 duced by two parents belonging to the Fi generation, and so on. 



