No. 637] 



FIRST GENERATION HYBRIDS 



119 



the character. Only a few of the more obvious of these 

 abnormalities have been studied, but there is no line of 

 demarcation between these conspicuous changes and 

 those that are less evident down to variations that can 

 not be distinguished visually from environmentally in- 

 duced fluctuations. 



Different varieties possess different assortments of 

 deleterious characters and in a cross between two unre- 

 lated strains all of the recessive lethal or semi-lethal 

 characters^ not common to both parents are kept from 

 expression, since the recessive characters of each parent 

 are brought into combination with, and suppressed by, 

 their dominant allelomorphs in the other parent. Freed 

 from the depressing effects of these recessive characters, 

 the first generation of a hybrid is usually more vigorous 

 than either parent. In subsequent generations the old 

 recessive characters again come into expression in some 

 of the plants, thus reducing the general vigor below that 

 of the first generation. 



If the above explanation of heterosis is to be accepted 

 it should follow that a majority of the departures from 

 the normal must be deleterious and recessive while those 

 which are advantageous are dominant. 



The existence, on the other hand, of advantageous re- 

 cessive, or deleterious dominant, variations would operate 

 to make Fj populations less vigorous than the average 

 of theii; parents and conversely inbreeding would tend to 

 increase vigor. 



Yarta i IONS IN .Maize Chiefly Deleteeious and Eecessive 

 Xoiu" of lh.' ivcordcd Mendelian variations of maize 



