68 



SCIENCE 



[N. S. Vol. XL. No. 1019 



the multiple factor hypothesis is indicated 

 clearly by these statements : 



We know that size characters do segregate in the 

 F;, but we admit that with them the simple Men- 

 delian ratio of 1:2:1 is never realized, though in 

 large populations the parental sizes may reappear. 

 Mendelians commonly try to account for the com- 

 plicated ratios by assuming the presence of mul- 

 tiple factors; non-Mendelians point to the same 

 ratios as quasi-evidence against Mendelian in- 

 heritance. I offer a different explanation. 



By way Of conclusion, Groth further re- 

 marks : 



The finding in the F, or later generations of lines 

 which breed true to size characters is thus not 

 proof of the presence of multiple size factors in the 

 original parents. 



It is evident, however, notwithstanding 

 Groth's disavowal, that his hypothesis is dis- 

 tinctly a multiple factor one. His suggestions 

 as to how spherical fruited parent races, the 

 dimensions of whose fruits are 4X4X4 and 

 9X9X9 respectively, might combine to pro- 

 duce Fj fruits of dimensions 6X^X6 is 

 rightly regarded as having a bearing " beyond 

 furnishing an explanation of partial domi- 

 nance in Fj." It might seem at first that he 

 regards volumes as the inherited units and 

 that volume, together with a shape factor, con- 

 trols linear dimensions. This is evidently not, 

 however, his idea. In the cross noted above 

 for illustration, a gamete bearing a length 

 factor 9, a breadth factor 9 and a thickness 

 factor 9 differs from a gamete bearing a length 

 factor 9, a breadth factor 4, and a thickness 

 factor 9 or 4 with respect to its effect not only 

 upon the volume of the resulting fruits but 

 also upon the length of those fruits. The pos- 

 tulated spherical shape factor, which is com- 

 mon to all gametes, but which modifies the 

 common length factor 9 only in case the 

 breadth or thickness factors are other than 9 

 and does not modify it in ease these breadth 

 and thickness factors are 9, is certainly some- 

 what confusing. But to say that a length 

 factor 9 produces an effect equal to 9 in 

 length when the breadth and thickness factors 

 are also 9 and produces some other effect on 

 length when the breadth and thickness factors 



are other than 9 is merely the equivalent of 

 saying that the breadth and the thickness 

 factors have an effect upon length and are 

 thereby length factors. This makes three 

 factors for length — a typical multiple-factor 

 hypothesis. 



Again, if the presence of the somewhat 

 fanciful shape factor be insisted upon, we are 

 still dealing with multiple factors. In his 

 illustration, Groth assumes two length factors, 

 4 and 9 and a shape factor that modifies them 

 under certain conditions. This makes three 

 factors affecting length. We can not limit the 

 length fact'ors to the two, 4 and 9, and say 

 that the third factor assumed to modify 

 length is nevertheless not a real length factor 

 merely because we have chosen to call it a 

 shape factor. Genetic factors for any char- 

 acter are the inherited units that have an 

 effect upon the development of that character. 

 The fact that some of them may also be con- 

 cerned in the development of other characters, 

 while really important, is immaterial in this 

 connection. 



It was said above that a shape factor affect- 

 ing length, plus the two length factors 4 and 

 9, make a complex of three multiple factors 

 for length. As a matter of fact there are more 

 than three such factors, if we hold to the shape 

 factor. The shape factor was shown to modify 

 length only in certain cases, namely, when the 

 breadth or the thickness factor is not of the 

 same value as the length factor. In other 

 words, the ability of a shape factor to modify 

 length is influenced by the presence of 

 breadth and thickness factors and the latter 

 thereby become at least indirect length factors. 

 But who, in the present state of our knowl- 

 edge, can say that the assumed primary length 

 factors 4 and 9 are less indirect in their effect 

 than are the other factors influencing length? 



I do not wish to appear too critical of 

 Groth's suggestions. It is only by a careful 

 analysis of such novel suggestions that we can 

 hope to gain a better understanding of how 

 genetic factors behave. My purpose is merely 

 to aid in such ati analysis. 



R. A. Emerson 



IjNIVEESinr OF Nebeaska 



