134 Outline of Genetics 



2. Inbreeding. 



(This again might be used as an example of opposite extremes 

 of causes producing the same end result, for inbreeding can surely 

 be regarded as the antithesis of wide crosses.) 



Inbreeding commonly results in "loss of vigor," through the 

 production of weakling and degenerate plants of various types 

 (see chapter on "Hybrid vigor"). Frequently these degenerate 

 types exhibit faulty and ineffective reproductive parts; Types of 

 this sort have frequently been obtained through inbreeding corn. 



3. Definite hereditary factors. 



A good example of this appears in the case of "tunicate" or 

 "podded" corn. Plants homozygous for the tunicate factor are 

 sterile, while the heterozygotes are partially sterile (Eyster 7). 



B. Semi-sterility. 



1. Wide crosses. 



The hybrids produced by wide crosses are not in all cases 

 completely sterile (see III, A, i), but merely show an abortion of 

 part of the gametes, notably part of the pollen. This phenomenon 

 is, in fact, of such general occurrence that the existence of a certain 

 amount of defective pollen is frequently used as a criterion of 

 hybrid origin. ]VIany plants in nature have been found to show 

 this characteristic; and such plants have been called "crypt- 

 hybrids," the impHcation being that they are hybrids that have 

 resulted from natural crossing. 



2. Inbreeding. 



Some of the degenerate plants that commonly appear as a 

 result of inbreeding (see III, A, 2) are not completely sterile, but 

 merely unsuccessful in setting more than a few seeds. This may 

 be due to a failure of part of the pollen or part of the ovules or part 

 of both. 



3. Definite hereditary factors. 



Corn which is heterozygous for the tunicate factor (see III, 

 A, 3) is partly sterile. 



Here also comes a very unique case, which will be described in 

 some detail, since it not only provides an ideal example of sys- 

 tematic semi-sterility through the operation of definite hereditary 

 factors, but at the same time it provides an example of inheritance 

 in the gametophyte generation. 



