SEX AND HEREDITY 473 



based, with Professor Punnett's permission, upon passages in his book 

 on Menclelism. 



In the selection of a plant for experiment Mendel recognised that two 

 conditions must be fulfilled. In the first place the plant must possess 

 differentiating characters, and secondly, the hybrids must be protected 

 from the influence of foreign pollen during the flowering period. In 

 Pisum sativum Mendel found an almost ideal plant to work with. The 

 separate flowers are self-fertilising, whilst complications from insect- 

 interference are practically non-existent. There are numerous varieties 

 of the eating-pea exhibiting characters to which they breed true. Mendel 

 selected a certain number of such differentiating characters, and 

 investigated their inheritance separately for each character. Thus in one 

 series of experiments he concentrated his attention on the stature of the 

 plants. Crosses were made between tall and dwarf varieties, which previous 

 experience had shown to come true to type with regard to these characters. 

 It mattered not which was the pollen-producing, and which the seed-bearing 

 plant. In every case the result was the same. Tall plants resulted from the 

 cross. For this reason Mendel applied the terms dominant and recessive to the 

 tall and dwarf habits respectively. Seeds collected from the first cross (Fi), and 

 sown the following year, gave both tall and dwarf plants among the offspring. 

 Each individual was either tall or dwarf, and no intermediates appeared. In 

 one series of experiments Mendel obtained 1064 plants, of which 787 were tall 

 and 277 were dwarfs. That is, the dominant and recessive characters occur 

 in the second generation of hybrids (Fa) in the proportion of 3 : i. 



In the following year seeds from this generation (Fa) were sown as before, 

 and produced F3 generation. From the seeds of the dwarfs came only dwarfs, 

 i.e. the recessive character bred true. The tall plants, however, of the F2 

 generation were not all of the same nature. Some of them produced seed 

 which gave rise to tall plants only. Others formed seed from which sprang 

 both tails and dwarfs in the proportion of 3 : i. The tall plants of the F2 

 generation thus proved to be of two kinds, viz. those which carried only the 

 tall character, and those which carried both tall and dwarf characters. The 

 former are called " pure," and the latter " impure " dominants. By observa- 

 tions on F3, and subsequent generations, Mendel showed that the pure domin- 

 ants and recessives always bred true, resembling in this way the original 

 parents. The impure dominants, on the other hand, always gave dominants 

 and recessives in the constant ratio of 3 : i. Since the pure dominants are 

 only half as numerous as the impure dominants, it follows that the impure 

 dominant, on being self-fertilised, produces as offspring pure dominants, 

 impure dominants, and recessives in the proportion of i : 2 : i. The case of 

 only one pair of characters has been considered here, but Mendel showed that 

 the rule holds good for all the various pairs of differentiating characters studied 

 by him ; and since his time his conclusions have been verified in numerous 

 instances both in Animals and Plants. 



A general scheme may be constructed showing the result of cross- 

 ing individuals which each bear one of a pair of differentiating char- 

 acters. If D represent the pure dominant ; and the impure dominant, 



