DOMINANCE AND THE VIGOR OF FIEST 

 GENERATION HYBRIDS 



G. N. COLLINS 

 Bureau of Plaxt Ixdustry, U. S. Department of Agriculture 



A STIMULATION of growth has come to be recognized as 

 one of the results of hybridization. The phenomenon is 

 of so much importance, practical as well as theoretical, 

 that it has been given a special designation, heterosis. 

 (Shull, 1914.) 



New interest has been given to the study of the causes 

 of this increased vigor by the work of Dr. Donald F. 

 Jones^ (1917 and 1918). Briefly stated the theory ac- 

 cepted by Jones is that growth is affected by a number 

 of different characters or factors, the dominant members 

 of each character pair being favorable and the recessive 

 unfavorable to growth. Each strain or variety possesses 

 some dominant and some recessive characters. When 

 two strains are crossed the first generation- of the hybrid 

 exhibits the dominant characters of both parents and is 

 in consequence more vigorous than either parent. In sub- 

 sequent generations the number of dominant characters 

 in any individual can not be greater than in the first gen- 

 eration and in a large majority of instances will be less, 

 hence the average vigor of the second generation, al- 

 though still above that of the parents, will be below that 

 of the first. 



The theory is not new, but has not been generally ac- 

 cepted because of outstanding objections. Dr. Jones has 

 reviewed the earlier M'Ork and has advanced a very in- 

 genious and entirely novel explanation of the objections. 

 This explanation will be discussed later. 



Bruce (1910) from purely mathematical considera- 



1 The theory has been further elucidated in the monograph on "Inbreed- 

 ing and Outbreeding" by East and Jones (1920). 



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