1 68 MAIZE 



CHAP, unit-characters ... is the first great debt that science owes 

 v - to Mendel " {Lock, i ). 



Segregation of unit-characters derived from the union of 

 two dissimilar gametes cannot take place in the first (Fj) 

 generation because only one zygote is formed by the union of 

 two gametes. However if the gametes are dissimilar, the 

 zygote is heterozygous, and in the second (F„) generation, 

 segregation of unit-characters does take place among the 

 many newly-formed gametes of the heterozygote. 



Segregation of unit-characters takes place prior to the 

 separation of the two germinal nuclei in the pollen tube (1 78), 

 since the two germinal nuclei of any pollen grain always bear 

 the same allelomorph (V 129) (Lock, I, p. 121). 



The completeness of the segregation may be still further 

 demonstrated by planting all of the grains from the F 2 ear 

 and selfing each plant. The progeny (the F 3 seed generation, 

 borne on the F 2 plant generation) will, if there are enough of 

 them, consist of nine distinct types of ear (Figs. 74, 75, and 

 76) as follows : — 



(a) The white wrinkled grains produce only white wrinkled 

 (Fig. 76G) ; they are therefore recessive (1T 129) both for white- 

 ness and for wrinkledness, and will breed true. These are 

 called extracted recessives. 



(b) The white flints will produce two sorts of ear: (1) a 

 few ears bearing all white flint grains (Fig. 76 I) some of 



which will probably breed true in the following generation ; 

 if they do, they will be pure dominants (IT 1 29), called extracted 

 dominants ; and (2) more ears producing both white flint 

 and white wrinkled grains (Fig. 7611) approximately in the 

 proportion of 75 per cent of the former and 25 per cent of 

 the latter, or 3 to I. 



(c) The black wrinkled grains will produce two sorts of 

 ear : (1) with all black wrinkled grains (Fig. 74c) some of which 

 will probably breed true in the following generation, in which 

 case they will be pure dominants, also called extracted domin- 

 ants. (2) Some ears will be borne which produce both black 

 and white wrinkled grains (Fig. 75F), in the proportion of 

 75 per cent of the former to 25 per cent of the latter. 



(d) The black flints will produce four types of ear: (1) 

 ears like the parent ear, carrying black flint, black wrinkled, 



