CHAPTER XXVII 



HYBRID VIGOR OR HETEROSIS 



Plants or animals which maintain normal size and vigor 

 under self-fertilization or close inbreeding may nevertheless 

 show an added vigor when outcrossed, that is when mated 

 with individuals of races genetically different from their own. 

 This is called heterosis, because it is supposedly due to 

 heterozygosis, the cross-bred state of genetic factors. The 

 mule has already been mentioned as a familiar example 

 among animals, in which hybrid vigor is shown. Many 

 similar examples are on record for hybrid plants. For ex- 

 ample East and Hayes describe a cross between two dif- 

 ferent wild varieties of tobacco {Nicotiana rustica brazilia 

 and N. rustica scahra) showing that reciprocal crosses pro- 

 duce Fi plants taller than either parent variety. See Table 

 326. 



TABLE 326 



Vaeiation en Height of Plants of Nicotiana rustica brazilia (349), of N. rustica 



scahra (352), and of their Reciprocal Fi Hybrids 



{After East arid Hayes) 



In maize, which is normally cross-fertilized and so main- 

 tained as a field crop in a state normally heterozygous, self- 

 fertilization for a number of generations serves automati- 

 cally to eliminate most of the heterozygosity (see Fig. 143) 

 and consequently produces races of size and vigor less than 



242 



