NUTRITION 



293 



it was found to be merely dormant, as it were, because it reappeared in the next 

 following generation. The tall individuals of the first generation were allowed to 

 fertilise amongst themselves only and, in the whole number of individuals produced, 

 there was found to be one dwarf to every three tails. The remarkable fact 

 is that the dwarfs are found to be of pure breed, that is, all of their posterity 

 are dwarfs. Of the tails, one-third are pure breed, the other two-thirds will 

 give the same mixture of one dwarf to three tails in the next generation and 

 so on. We shall see presently how this proportionality is to be explained. 

 For the present, we note that, although they may be the offspring of two 



& 



Fio. 78. PHOTOGRAPH OF SWEET PEA PLANTS. Five tall and three dwarf, grown from the 

 same pod from a cross between a tall and a dwarf variety. The dwarf plants on the left 

 are pure breed, recessive, although one of the parents was tall. The tall character is 

 the dominant one, and some of the tall individuals are mixed breed. The whole family 

 corresponds to the stage F 2 of Fig. 79. 



(Bateson, 1906, PI. 1.) 



tall individuals, the dwarfs are pure breed (Fig. 78, from Bateson's Address, 1906, 

 illustrates some of these facts). 



Now, in the sexual cells, or gametes, male and female, the chromosomes, 

 arising from the nucleus by mitotic division, are only half the number of those 

 formed in the somatic or general body cells ; so that, when the sexual cells 

 unite to form a zygote, the normal number is again attained. Taking, for 

 simplicity, the characters of blackness and whiteness whose heredity is to be 

 traced, the scheme of Fig. 79 will represent matters. 



It is to be remembered that blackness and whiteness do not always obey simple Mendelian 

 laws, so that any character taken at haphazard cannot be used to trace the laws of heredity. 



We have just seen that, compared with the germ cells, the body cells are 

 double structures, so that, for the present purpose, we may represent a pure 

 black individual by two black symbols, say rectangles, each rectangle representing 

 a germ cell. A pure white individual is represented by two white rectangles. 

 Further, when a black and white individual are crossed, the hybrid contains 



