l68 AN INTRODUCTION TO MODERN GENETICS 



Drosophila melanogaster, have been shown by Muller^ to have different 

 efficiencies in producing red pigment, i.e. to be on different parts of the 

 dose-effect curve. (Cf. pp. 185, 298.) 



9. Multiple Allelomorphs 



The simplest series of multiple allelomorphs are related to one 

 another as hypo- and hypermorphs. That is, they all produce the same 

 kind of effect but with different efficiencies. The genes at the locus of 

 white eye in Drosophila melanogaster are a good example. These range 

 down through coral, which is very little hghter than the normal red 

 eye, through a whole series of members (eosin, cherry, apricot, buff, 

 tinged, ivory, etc.) to white, which is practically an amorph, although 

 not quite, as white-deficiency is even lighter in shade. Stern^ in par- 

 ticular has stressed this quantitative relation between the effects of 

 different members of an allelomorphic series (see p. 164). He has also 

 drawn attention to the fact that simple relations of this kind, though 

 common, are not by any means universal. Thus in the white series 

 itself, there is the remarkable fact that the series of diminishing effec- 

 tiveness is different for different phenotypic actions of the genes. ^ 



Eye colour . . . . W > w^ > w^ > w 

 Vitality . . . . W > w"" > w^ > w 



FertiUty . . . . W > w > w^ > w^ 



In other cases it may be impossible to arrange the allelomorphs in 

 series because their effects are non-commensurate, e.g. the locus of 

 spineless has two allelomorphs, ss giving bristle reduction, and ss° 

 (aristopoedia) giving foot-like appendages in place of the arista in 

 Drosophila melanogaster. 



Another example in the same organism is the dumpy series.^ 

 This locus has three main effects: shortening the wing, causing the 

 formation of vortices of the hairs on the thorax, and reducing the 

 viability. Nearly all the possible combinations of these effects can be 

 found in different allelomorphs, as is shown in the table, in which 

 abnormal effects are indicated by minus signs, the number of which 

 gives an indication of their strength. 



1 Muller 1933. 2 Stern 1930. 



^ Timofeeff-Ressovsky 1933a. 



* Muller (personal communication), cf. Stern 1930. 



