214 GENETICS 



5. HETEROSIS 



When hybrids are formed the first hybrid generation / 

 not only shows more variability but also more vigor \ 

 than the parental strains and this vigor is in proportion k 

 to the number of factors in which the parents differ, be- 

 cause in hybridization there is a pooling of hereditary 

 resources. Such hybrid vigor is termed hetergsis. 



East and Hayes describe, for example, a cross be- 

 tween two different wild varieties of tobacco in which 

 the average height of over fifty plants of each of the 

 two wild parents was 31 and 54* inches respectively. 

 Of an equal number of hybrids of the first generation 

 the average height was over 67 inches under the same 

 environmental conditions. Shull and East, working 

 separately upon maize, came to the same conclusion, 

 namely, that the first hybrid generation following an 

 artificial cross is decidedly more vigorous than the 

 parental stocks from which it is derived. This is shown 

 in Figures 45 and 46. 



The mule is a notorious hybrid that possesses more 

 "kick" than its parents. 



