574 BOTANY OF THE LIVING PLANT 



The fundamental assumption made by Mendel was that of the 

 " purity of the gametes " ; this still holds good, except in the case of 

 polyploids (see below, p. 581), where both allelomorphs may occur in 

 the same gamete. The two subsidiary assumptions, which are also 

 borne out by observation, are (1) that the different kinds of gametes 

 are produced in equal numbers ; and (2) that these gametes mate at 

 random. 



Genetics. 



For various reasons the far-reaching importance of the simple 

 principles laid down by Mendel on the basis of his own experiments 

 was not recognised for more than thirty years. The re-discovery of 

 his work in 1900 gave a fresh stimulus to the experimental study of 

 inheritance, and since that date this field of investigation has devel- 

 oped to a remarkable extent so as to constitute a separate province of 

 research known as Genetics. Much of this expansion has been due to 

 the discovery of the chromosomes (unknown in Mendel's day) and to 

 the increasing precision in our knowledge of the mechanism of nuclear 

 division and in particular of the details of meiosis. At the present 

 time genetics and cytology are indissolubly linked, and the mcst 

 striking advances in the study of inheritance have been achieved by 

 " cytogenetical " methods. Only a few of the more generally impor- 

 tant developments in genetical knowledge will be mentioned here. 



Examples of Segregation in Hybrids. 



(a) Involving One Pair of Characters. 



Mendel's classical experiment with tall and dwarf Peas which has been 

 discussed in detail above, illustrates what happens in a " monohybrid " 

 cross, i.e. one in which the parents differ in only a single Mendelian character. 

 In the case described, the tall habit is completely dominant to the dwarf 

 habit, so that the " hybrid tall " (heterozygous) individuals of the F 2 genera- 

 tion are not visibly distinct from the " pure tall " (homozygous) plants. 

 Such complete dominance is by no means universal ; in many other cases 

 the F x individuals are more or less intermediate, in respect of the differ- 

 entiating character, between the two parents. Thus, if a true-breeding 

 crimson-flowered Snapdragon {Antirrhinum majus) is crossed with a true- 

 breeding ' ivory-flowered " (cream-coloured) individual (Fig. 437), the 

 F x plants all have pink flowers. The F 2 generation segregates into 25 per 

 cent, crimson, 25 per cent, ivory, and 50 per cent, pink-flowered progeny. 

 The crimson and the ivory individuals are homozygous ; the pink-flowered 

 plants are heterozygous and on being " selfed " segregate just like the F x 

 plants. Because of this incomplete dominance of crimson over ivory in the 



