146 Sl'»ll- 



This indicates that the deficieucy in the Heegeri individuals, pl■e^^ously 

 reported, was probably due, in larg:e part at least, to selective elimination. 



In one family there was an e(|ual deficiency in the proportion of 

 recessives and of homozygous dominants, so that the ratio DD : DR : KR 

 was about 1:4:1 instead of \ :•> : \. Several possible interpretations 

 of this result are considered: namely, a) that it is a purely chance 

 result; b) that a selective fertilization has favored the union of unlike 

 gametes; c) that selective elimination has affected positive and negative 

 homozygotes equally because they lack the \\gov pi'oduced by hetero- 

 zygosis; and d) that some form of gametic coupling accurs between the 

 two genes C and D. Of these hypotheses a) and d) appear most promising. 



The discovery by Nilsson-Ehle and East that the same character 

 may be i)roduced by any one of several independently inheritable -de- 

 terminers, marks an important advance in genetic progress because it 

 has led to a well grounded Mendelian iuten)retation of inheritable quan- 

 titative differences. Determiners which independently produce such 

 equivalent results, I have called "duplicate" genes. 



A sharp distinction must be maintained between ^'duplicate" 

 determiners and "2)lnraV' determiners, the latter including all determiners, 

 of whatever nature, which produce a given characteristic or which modify 

 it in any way that does not destroy its identity. Inheritable quantita- 

 tive characters are probably the product of plural determiners, but not 

 to any considerable extent of duplicate determiners, and the develop- 

 ment of an adequate Mendelian interpretation of the inheritance of such 

 characteristics need not have awaited the discovery of duplicate determiners. 



For only four characters is the evidence of duijlicate genes to be 

 deemed adequate: namely, for the presence of a ligula in oats, red 

 pericarp-color in wheat, yellow endosperm-color in maize, and the tri- 

 angular capsule-form in Bursa. For many other characters the existence 

 of ji/ural determiners has been rendered pro])able, but there are weaknesses 

 in the evidence, and apparently fallacious inferences have been drawn 

 ill ;i iiiiinlier of cases, owing to tlie failure to distinguish between plural 

 and duplicate deteniiiners. 



l'"(ir many chai-acteis the only evideiire (if plui'al Mendelian si/.e- 

 deti'iiiiiiiers is an increased variability in l'"i as cdiiipared witli the I'l 

 and Fl generatiims. It is iiere shown: a) that the stimulating ellect of 

 hybridity, for which the name 'iieterosis" is adopted, produces increased 

 variability in ([uantitative cliaiaetiMs in the F2 and a pseudo-segregation 

 in F.S, indejiendently of the existence o| specific determiners for size; 

 bi that tlie di'iiionstration of an increased variability in F2 does not by 



