1916] Beeves: Inheritance of Extra Bristles in Drosophila 507 



since crosses between extras have given always a large majority of 

 normals, it is impossible to speak of dominance in the ordinary sense. 

 If we assume that one or more factors are concerned in the appearance 

 of extra bristles, it is not necessary to resort to the idea of the loss of 

 an inhibiting factor to explain the increased number of extra-bristled 

 individuals in selected generations. According to the law of chance, 

 one naturally would not expect all three or even two factors to be 

 present in every mating. Thus we might perhaps expect to find some 

 normals present in all cultures, even those that have been most rigor- 

 ously selected. This, however, involves also the admission that the 

 factors themselves are more or less inconstant, at least in their expres- 

 sion in the soma. 



2. Variation. — From a viewpoint other than Mendelian the per- 

 centages of extras (tables III, IV, V, and VI) seems to indicate a 

 fluctuating variation or sliding scale, as the percentages vary from 

 1.1 to 15. Additional support of this idea is found in the fact that 

 extras appear in all cultures. Where normals are crossed with nor- 

 mals (table I, y F2) the percentage of extras is 11.11, exactly the same 

 as when extra is crossed with extra (table VII, F^Xe), 11.11 per cent. 

 This seems to point to the fact that we are dealing either with an 

 ever-varying factor or with germ plasm in such unstable balance that 

 it has a continual tendency to vary. 



In the A family in the F^ generation, five varying flies were found, 

 three having five bristles and also two with three. We might conclude 

 from such facts as these that the germ plasm is in an unstable con- 

 dition, with a tendency to vary and to vary in either direction. 



3. Partial Inheritance. — Instead of all flies inheriting the extra 

 bristle, a tendency to partial inheritance was found. These inter- 

 mediates had, instead of a definite extra bristle, merely a thickened 

 hair. Such thickened hairs appeared in line with the bristle and in 

 positions where one might expect to find an extra bristle. 



TABLE XIX 

 Eesults of Mating Different Sibs of the Same Families 



Family 



Bottle 



letter 



Normals 



Extras 



Per cent 



Y 



E- 



pair 



100 



3 



2.9 





E^ 



pair 



77 



3 



3.8 





E 



mass 



60 



6 



9.9 





E 



pair 



67 













E 



pair 



95 



2 



2.07 



X 



A 



pair 



98 



5 



2.8 





A 



pair 



75 



1 



1.3 





A 



mass 



85 



3 



2.3 



