The Behavior of the Cliromosonies as Studied through Linkage. 265 



fallacy arises from the prevalent "presence and absence"' theory, which 

 is itself another good example of a theory regarding physiology which 

 is unnecessary and not founded on sound evidence (see Mokg.Ov' '13 a, 

 Sturtevant '13 c). 



Although there is little that we can say as to the nature of Men- 

 delian genes, we do know that they are not "determinants"' in the 

 Weismanuian sense. This is well shown by the following case. The 

 difference between normal red eyes and colorless (white) ones in Droso- 

 phila is due to a differenci- in a single gene. Yet red is a very complex 

 color, requiring the interaction of at least five (and probably of very 

 many more) different genes for its production. And these genes are 

 quite independent, each chromosome bearing some of them. Moreover, 

 eye-color is indirectly dependent upon a large number of other genes, 

 such as those on wliich the life of the fly depends. We can, then, in 

 no sense identify a given gene with the red color of the eye, even 

 though there is a single gene differentiating it from the colorless eye. 



So it is for all characters — as Wilsox ('12) has put it " the 



entire germinal complex is directly or indirectly involved in the production 

 of even- character."' All that we mean when we speak of a gene for 

 pink eyes is, a gene which differentiates a pink eyed fly from a normal 

 one — not a gene which produces pink eyes per se, for the character 

 pink eyes is dependent iipim the action of many other genes. 



On the other hand, the jft-tion of any one gene is not necessarily 

 limited to one somatic character. Thus in Drosopliila the gene which 

 differentiates rudimentär}' winged flies from normal ones also has the 

 following effects when not counteracted by its normal allelomorph: it 

 causes the flies that bear it to be less viable in the stages of develop- 

 ment before the production of the winged adult ; it causes the females 

 bearing it to produce relatively few offspring; and it acts in such a 

 way that flies bearing it produce no offspring when mated together. 

 Moreover, the rudimentary wing itself differs from the normal in size, 

 in shape, in the arrangement of the veins, and in a tendency toward 

 "ballooning" or blistering. Further, rudimentary winged flies usually 

 have abnormal legs. Numerous similar instances will occur to everyone 

 familiar with Mendelian phenomena. 



We may conclude, then, with certainty that at least most charac- 

 ters are dependent upon the interaction of numerous genes: and, on 

 the other hand, that most, or perhaps all, genes affect many different 

 characters. 



