GENIC INTERACTION 



173 



number of dorsal rosettes varied in both directions. It might be at least doubled 

 (grade A ++ ) or it might be reduced to two (grade AC). In cases of asymmetry, 

 rosettes again tended to occur more on the right than on the left side. 



Genes M, m are specific modifiers of R with no detectable effect in its absence. 

 Other specific modifiers were clearly present. As indicated above, high grade RMm 

 (grade CA) differed markedly from low grade Rmm (grade AC). It appears that the 

 number of dorsal rosettes was especially subject to modifiers, 2 [MRd), in both of 

 these genotypes. The effects of these modifiers were, however, confused by a very 



Fig. 27. Diagram of factor interactions in the determination of hair direction and 



PLEIOTROPIC EFFECT ON WHITE FOREHEAD SPOT IN THE GUINEA PIG. 



StSt>Sts1 St St 



RR-Rr 



R r / 

 — - / 

 i 

 i 



MM (I) / 



Mm (1,2) u /— 



mm (1,2,3) M m 



-X 



/ 



Growth pattern 

 n skin 



ReRe>Rere Re rj £(MRe 

 C 



Melanoblast 



/ \V//' Forehead--' 



Process //A ~v 6] 

 /// /\ 



-\.( 2 )-^Back^[*Mid 



R " 



[3LAnterior 



Process 



(3) Posterior 

 Hind feet 

 Fore feet 



White spot 



considerable amount of nongenetic variability. Thus the regression of offspring on 

 midparent in matings of Rmm x Rmm, including an excess of the extremes AC and 

 A ++ , was only 0.48, indicating that only about half of the variability was genetic. 

 Three generations of selection did not suffice to fix grade AC. 1428, 1429 



As indicated above, eye rosettes ordinarily appear only in roughs of high grade 

 (Rmm). Strong eye rosettes were, however, recorded in four individuals that were 

 otherwise of grade E and of genotype RMM, from separate matings that were only 

 remotely related and had no record of the character in their ancestry (other than in 

 grade A). A considerable number (171 born alive, 21 born dead) were derived from 

 one of these lines but a complete analysis of the genetics was not made. There are, 

 however, a number of points of interest with respect to interaction in the unpublished 

 data. 



The character proved to be a very troublesome one because of intergradation 

 with the condition in normal, smooth-furred animals (rr) which themselves show a slight 

 divergence of hair direction about the eyes. Among those considered to have the 



