GENIC INTERACTION 175 



ally strong forehead rosette might have been expected by combination of the effect of 

 Rmm and of St, there was actually only a slight irregularity of hair direction on the 

 forehead. The anterior dorsal rosettes of grade A were weak and lateral in position, 

 leaving a broad smooth shield anteriorly on the back. This last feature refers to 

 RmmStSt. In this case only, St ceased to be completely dominant and RmmStst showed 

 a great variety of intermediate conditions of the dorsal rosettes. It is interesting that 

 RMM and RMm tend to inhibit the effect of St in a part of the coat, the forehead, in 

 which they have no visible effect in the presence of stst and that St in turn inhibits the 

 effects of RMm and Rmm in a region, the anterior back, in which it has no visible effect 

 in the absence of R. In view of these effects, it ceases to be surprising that Rmm and St 

 interact to prevent development of the forehead rosette which each determines by itself. 

 Gene St has an interesting pleiotropic effect in a tendency toward a white spot in 

 front of the center of the forehead rosette. It was shown by Bock 114 that this does not 

 require even heterozygosis in the spotting factor and that it tends to be inhibited by R, 

 especially Rmm. Table 42 gives a condensed summary of his results for young with no 



Table 42 



Percentage of white forehead spot in guinea pigs with no white outside of 



the forehead 



According to grades of roughness due to Rr, Mm and to presence (St) or absence (stst) of 

 forehead rosette from matings that involved St. 



white outside of the forehead and from matings that involved St. The results indicate 

 that the white forehead spot is a secondary consequence of the effect of St on the skin 

 of the forehead, manifested in the forehead rosette. The neutralizing effect of R on 

 the latter, especially in the combination R-mm, also tends to neutralize the white spot- 

 ting. Another element of the full rough pattern of Rmm, the feathering on the belly, 

 has also been observed independently of R in a certain somewhat inbred strain. 1416 

 It followed an irregular course of inheritance, neither a simple dominant nor a simple 

 recessive. Dorsal irregularity of hair direction has also occurred independently of R 

 but so sporadically that mere developmental accident seemed indicated. The most 

 important general result of this study of the genetics of hair direction seems to be another 

 revelation of the extraordinary diversity in the relation of genes to characters, and in 



