DONALD F. JONES 



Connecf/'cuf Agricultural Exper'imenf Sfaiion 



Chapter 14 



Plasmagenes and 

 Chromogenes in Heterosis 



The word heterosis is essentially a contraction of the phrase stimulus of 

 heterozygosis. It was first used by G. H. Shull (1914). The concept of a 

 stimulation resulting from the genetic union of unlike elements was de- 

 veloped by East (1909). Previous to the Mendelian conception of units of 

 heredity, it was generally considered by plant and animal breeders that the 

 invigorating effect of crossing unlike varieties of plants and breeds of live- 

 stock was due to the correction of imperfections that existed in both parental 

 types. This idea is clearly stated by Samuel Johnson in the second edition of 

 his book How Crops Grow (1891). 



The early recordings of instances of hybrid vigor and the various means 

 of accounting for this phenomenon have been stated and restated so many 

 times that there is no need or useful purpose in repeating them here. Excel- 

 lent reviews of the literature are readily available (see especially East and 

 Hayes, 1912; Jones, 1918; East and Jones, 1919; East, 1936; and Whaley, 

 1944). 



THE EXPRESSION OF HETEROSIS 



At the present time, the term heterosis designates the increased growth or 

 other augmented action resulting from crossing, however it is produced. As 

 generally used, it is essentially synonymous with hybrid vigor. Heterosis has 

 two general modes of expression. In one, there is an increase in size or num- 

 ber of parts. This is usually the result of a greater number of cells and a faster 

 rate of cell division and cell activities. This results in an improvement in gen- 

 eral well-being of the organism similar to the result of being placed in a more 

 favorable environment. Such luxuriance may be accompanied by partial or 

 complete sterility in diverse crosses. 



A somewhat different manifestation of heterosis is an increase in bio- 



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