THE MUTATED GENE 161 



gene which prevents pigmentation in certain areas (see Onslow, 

 page 91). There is a considerable influence of nongenetic 

 factors on the variability of the character, studied especially by 

 Wright et al. in guinea pigs. There is further the fact that 

 pigmentation in these cases is dependent upon the temperature 

 and may be changed considerably, especially in the so-called 

 Dutch pattern of pigmentation of the tips (ear, nose, feet) by 

 temperature action. Actually, exposure to cold leads to pig- 

 mentation of otherwise unpigmented parts (W. Schultz, see 

 page 155). There is a considerable amount of genetic modifica- 

 tion of the pattern, i.e., the extent of the white areas. Modifying 

 genes may be accumulated by selection and result in almost 

 self-colored or almost white condition. (Most of the literature is 

 found in the papers of ( astle, Wright, Dunn). In a recent paper, 

 Dunn and Charles (1937) showed that, in mice at least, other 

 genes for spotting exist, which cannot be called modifiers, because 

 in the absence of the main gene they also cause spotting. These 

 give a combination effect with the main gene, and the cumulative 

 action of the different spotting genes determines the phenotype. 

 This, then, is a case of multiple-factor action, known in innumer- 

 able cases of quantitative characters and mentioned above also in 

 connection with pattern types in the case of the nun moth (page 

 128). In the mouse case, this cumulative action may appear 

 phenotypically as a change of dominance of the main gene: S/s 

 (s is the recessive spotting gene) is Wild type; in the presence of 

 another spotting gene (of the iC-group), spotting is somewhat 

 dominant. Of course, this is not a shift of dominance but an 

 additive effect visible also in the heterozygote. 



These facts lead to the following deliberations. The inter- 

 action of different genes in controlling the phenotype may 

 oscillate between two extremes. 



1. The genes in question exercise their main effects upon 

 different reactions which do not interfere with each other. In 

 this case, the phenotype will show a combination of the different 

 traits involved. Such cases have been worked out quantitatively 

 by Csik (1934a) for wing characters in Drosophila, many of which 

 may be independently combined without interfering with each 

 other. Such an effect may occur on the basis of qualitatively 

 different reactions or in connection with actions at different times. 

 But it is probable that such cases will prove to be rather rare (in 



