172 MAIZE 



CHAP, (c) that where unit-characters are opposed to each other, i.e. 

 belong to an allelomorphic pair, only one of them appears in 

 the immediate progeny ; this does not mean that the two 

 merge, for they segregate again in the second generation ; 

 (d) that parents markedly differing from each other in characters 

 which are not allelomorphic, will, if mated, produce in the 

 second generation offspring in which these characters will be 

 united in combinations differing from those of either parent. 



Cross-breeding is based on the principle that the union 

 between two dissimilar plants or animals will produce some- 

 thing dissimilar to either, though combining some of the 

 characters of each, as in the case of the black flint maize 

 produced by crossing a black wrinkled with a white flint, de- 

 scribed in IT 128. 



" The complete segregation of the two allelomorphs in 

 equal numbers of the germ cells of a heterozygote constitutes 

 the first and most important section of the generalization 

 known as Mendel's Law. The second part of the law refers to 

 the fact that, as a general rule, separate pairs of allelomorphs 

 segregate quite independently of one another" (Lock, I). In 

 other words, the gametes formed by a heterozygote contain 

 in equal numbers the pure parental allelomorphs completely 

 separated from one another ; and if the cross-bred plant is 

 heterozygous in respect of more than one pair of allelomorphs, 

 then all possible combinations of these allelomorphs occur in 

 equal numbers of gametes. 1 



I 30. Dominant and Recessive Allelomorphs. — The following 

 characters in maize have been found to behave as Mendelian 

 dominants and recessives respectively : — 



Dominant : Recessive : 



Starchiness of endosperm. Wrinkled (i.e. non-starchy) endo- 



sperm. 

 Flintness of endosperm. 2 Dentness of endosperm. 



Colour of endosperm (e.g. yellow," Absence of colour ; in a starchy 

 red, purple, or black). grain this shows as white ; a non- 



1 But allowance should be made for such exceptions as occur in the case of 

 coupling and repulsion of characters (IT 132). 



2 In some cases observed by the writer ; East and Hayes illustrate cases in 

 which the F x grains were intermediate in character, and in the F 2 the flint was 

 recessive in proportions indicating a dihybrid composition. 



:! There is a second yellow endosperm character which behaves as though its 

 appearance were dependent on the presence of a factor for colour ; in the absence 

 of this second factor the endosperm is white. 



