172 PHYSIOLOGIC GENETICS 



reaction was, however, moderately strong in frozen sections and indistinguishable in 

 pale cream and yellow areas of 14 tortoise shells of genotype e p e p Cppjf (W. L. Russell, 

 unpublished results of F. Appel and of L. B. Russell). These reactions were, however, 

 significantly less than in pale sepias (ECppF) and reds (eeCF). It appears that in the 

 eumelanic series F(in ppF) and even /(in ppff) substitute for P somewhat more 

 effectively in oxidising added tyrosine or dopa than in producing natural melanin. 



Replacement of B by bb causes no reduction in tyrosinase or dopa oxidase activity 

 by any test. There is, indeed, considerable evidence that the reactions are stronger 

 with bb, even in the extreme dingy browns. It appears that B, b takes no direct part 

 in the enzyme system responsible for oxidising added tyrosine or dopa. Perhaps 

 again greater utilization in blacks than in browns is associated with greater loss. 



CONCLUSIONS ON COLOR INTERACTIONS 



It may be seen that the products of the color factors enter into a rather compli- 

 cated pattern of interactions before formation of the final products: sepia, brown, or 

 yellow pigment granules. The pattern resembles somewhat the branching chains of 

 reactions worked out for gene-controlled metabolic reactions in microorganisms. 

 Some of the steps may indeed be of similar nature, since most of the factors probably act 

 in the nuclei of the pigment cells themselves as demonstrated in mice by Reed and 

 Henderson. 1044 Others, however, probably act from adjacent epidermal cells as al- 

 ready noted. The difference between adult male and female dingy browns is probably 

 under endocrine control. Some seem to be concerned directly with the pigment 

 process but others undoubtedly affect pigment merely because of effects on the vitality 

 or metabolic efficiency of the pigment cells. 



HAIR DIRECTION 



We will consider next a morphologic character, hair direction (figure 27) . In wild 

 cavies, the hair is directed away from the snout on the body, with minor qualifications, 

 and towards the toes on the legs. On introducing the dominant gene R into a genotype 

 that is otherwise like that of the wild species (rrMMrerestst) there is more or less reversal 

 on the feet, especially the hind feet (grade E) and occasionally a slight crest along the 

 back. 1428 ' 1429 Those with RMm are highly variable, but most of them have either 

 a strong dorsal crest (grade D) or a single pair of rosettes, halfway along the back 

 (grade C). Some of those called grade C had only a single dorsal rosette. This was 

 almost always (96 per cent) on the right side, a curious example of regular asymmetry. 

 A few were called grade E and many in certain strains were called grade B, or better, 

 CA, with two pairs of dorsal rosettes (as in grade A) but no rosettes on head or belly 

 (as in grade C) . With Rmm, there are typically two strong pairs of dorsal rosettes, 

 anterior and posterior, radiation from the ears, a strong forehead rosette, eye, check, 

 and groin rosettes, and feathering along the midline of the belly (grade A) . The 



