58 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



absence a, the other B, and its absence h, the first-generation hybrid 

 is constituted thus, AaBh. This hybrid produces four types of 

 gametes, namely, AB, Ab, aB, ab. An ovule of the type AB uniting 

 with pollen of the type AB gives AABB, a homozygote strain in 

 which the new character is fixed. While such cases as this are not 

 common, they may occasionally represent important advances in 

 breeding. It is therefore well for the breeder to understand them. 

 Several such cases have been found. They also probably represent 

 reversions to lost characters, at least in most cases. 



Characters may also appear in the second generation of a hybrid 

 that were not apparent either in the first generation or in either of 

 the parents. This is especially the case when a character is hypo- 

 static in one of the original parents of the cross; that is, when it is 

 covered up or hidden by some other character. A case in point is 

 the appearance of brown beans in the second generation of the cross 

 between black and white, reported by Shull. Here the brown is 

 hypostatic to black, i. e., obscured or hidden by the black, in the 

 black parent. Letting B represent black, h its absence, D brown, 

 and d the absence of brown, the formulae for the black and white forms 

 and the hybrid between them is — 



Black, BBDD. 

 White, bbdd. 

 Hybrid, BbDd. 



The gametes produced by this hybrid are BD, Bd, hD, and hd. 

 The union of an ovule of the type hD with pollen of the same type 

 gives hhDD, a brown type. 



Similar cases are known in animals. 



POSSIBILITY OF ENTIRELY NEW CHARACTERS. 



While most apparently new characters that arise in crossing are 

 probably reversions to lost characters, it is easily conceivable that 

 entirely new characters might arise in this manner. It seems prob- 

 able that some cases of reversion are due to reaction between chem- 

 ical substances, one of which is derived from one parent and the 

 other from the other. These substances are probably produced in 

 the cells of the respective pure strains before the cross; but they 

 produce no effect because they are not both present in the same 

 cells. It is conceivable that in some races there may have occurred 

 evolutionary changes that result in considerable modification of the 

 chemical contents of the cells but which produce no visible effect 

 on external characters. In two related races these evolutionary 

 changes may be quite different, and when we cross two strains 

 that have been separated for some thousands of generations we 

 may get, by reactions between substances that in the respective 



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