EYE COLOR IN DKOSOPHILA MELANOGASTER 377 



determine in which chromosome the gene for a given modifier 

 Hes. This information is therefore lacking in the case of cream 

 a, dark, and whiting. Of the five other modifiers, one, cream 

 III, is in the third chromosome, and the other four are in the 

 second. The presence of four modifiers of eosin eye color in the 

 second chromosome is in contrast to, or perhaps supplementary 

 to, the relative fewness of ordinary eye-color mutations in that 

 chromosome. None of the modifiers were sex-linked, but this 

 is probably not of special significance. 



Locus. The locus within the second chromosome has been 

 determined for two of the four second chromosome modifiers 

 of eosin, namely, pinkish at about 106, and cream b at 22.5. 

 The locus of cream III has not been found directly because of 

 the presence of Cm in the cream III stock. In heterozygous 

 Cm there has been found to be 4.2 per cent of crossing over 

 between cream III and dichaete, and 5.4 per cent of crossing 

 over between cream III and ebony. This corresponds to a locus 

 on the normal map of about 3.1 units to the left of dichaete or 

 at about 7.9 units to the right of sepia which is the zero-point 

 of the map of the III chromosome. 



DISCUSSION 



The facts of the inheritance of these specific modifiers show 

 that each is a definite Mendelian gene on the same footing with 

 the whole body of genes known in Drosophila. Thus, they 

 display clean-cut segregation from their allelomorphs; each is 

 located in and transmitted by a specific chromosome of the 

 Drosophila complex; they give free assortment with genes located 

 in other chromosomes ; and with genes located in the same chro- 

 mosome they show linkage, with crossing over corresponding to a 

 fixed locus. Each of these genes arose by mutation — "a specific 

 change in the hereditary constitution" — by the transformation 

 of the materials of a particular locus into a new form having a 

 different effect upon the developmental processes. 



These specific modifications are clear and simple cases of 

 'multiple genes.' Each is the result of the coaction of a specific 



