Duplicate genes for capsule-form in Bursa bursa-pastoris. _ ]33 



existing, but is a "creative force", which, by modifying unit-characters, 

 produces something otherwise unattainable. Leaving aside the faulty 

 logic which requires that something can be selected that is not first 

 presented for selection, the best explanation of the case appears to be 

 that this hooded color-pattern is really a complex character instead 

 of a simple one, the genotypic basis of which consists of one gene 

 having a large and fundamental effect in determining the general nature 

 of the hooded-pattern, and a number of other independently inheritable 

 factors which act as slight plus- and minus-modifiers of the action of 

 this fundamental gene. As these modifiers are not in any sense duplicates 

 of the fundamental pattern-factor itself, we are not driven, as CASTLE 

 says we are, as "the only logical escape" from the dilemma presented 

 by the invariable occurrence of a monohybrid ratio, "to assume further 

 that the assumed multiple units are all coupled." 



Another case in point concerns the inheritance of heights in maize 

 (EMEESON 1911). In two of EMERSON'S hybrid families there was a 

 sharp segregation into tall and dwarf plants, apparently due to the 

 presence and absence of a single Mendelian determiner, while in other 

 maize- crosses the F 2 presents a continuous series of height-differences 

 which suggest the presence of several independent determiners af- 

 fecting the height. EMERSON accepts CASTLE'S "only logical escape" 

 and assumes that in the apparently monohybrid families there was prob- 

 ably "a coupling of the several height characters." As EMERSON him- 

 self says, he is led to this construction by the consideration that "it 

 would seem more reasonable to suppose that similar differences in height 

 are due to a similar number of height characters." Here again lurks 

 the idea that these plural genes for height are duplicate genes. To 

 me it seems more reasonable to suppose that the height-modifiers are 

 generally not duplicate, and that consequently it is not illogical to 

 expect that some of them will produce much larger height-differences 

 than others. The possibility of coupled height-differences is not denied, 

 of course, for it is not improbable that coupling occurs among the genes 

 for quantitative differences as frequently as among those of fully 

 analyzable Mendelian characters, but coupling should not be assumed in 

 any case so long as a simpler interpretation is available. 



In connection with studies of plural determiners, HAYES (1912, 

 p. 22) and TAMMES (1913) have used the biometric coefficients of cor- 

 relation as indicative of the genotypic relations among the several 

 quantitative characters, but such correlations are so nearly universal and 

 are due to such a conglomeration of different causes that their use as 



