1 8 MENDELISM CHAP. 



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. 1 



Mendel experimented with other pairs of con- 

 trasted 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 dominant to green, and round 

 shape was dominant to wrinkled, 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. 

 dominants 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 characters with which he 

 .worked, Mendel set himself to providing a theoretical 

 (interpretation of this scheme which, as he clearly 

 /realised, must be in terms of germ cells. He con- 

 ]ceived of the gametes as bearers of something capable 

 /of giving rise to the characters of the plant, but he 

 /regarded any individual gamete as being able to 

 carry one and one only of any alternative pair of 

 I characters. A given gamete could carry tallness or 

 dwarfness, but not both. The two were mutually 



1 It has been found convenient to denote the various generations 

 resulting from a cross by the signs F I; F 2 , F 3 , etc. Fj on this system 

 denotes the first filial generation, F 2 the second filial generation pro- 

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



