162 PHYSIOLOGIC GENETICS 



COAT COLOR OF THE GUINEA PIG 



Figure 22 is an interpretation of the interaction pattern with respect to quality, 

 intensity, and pattern of the coat color of the guinea pig. Color depends on the pres- 

 ence of pigment granules (sepia, brown, or yellow) or their absence (white). The pig- 

 ment granules are produced in special cells (melanocytes) in the hair follicles, basal 

 layer of the epidermis, chorioid coat of the eye and certain other tissues, and in retinal 

 cells. All but the last migrate from the neural crest. The melanocytes may pass the 

 pigment granules to epidermal cells. Absence of pigment may be due to failure of the 

 melanocytes to reach the normal site, 1177a ' 1352a to death in the site, probably in silvering 

 and grizzling (at 1, 2, and 4, respectively, in figure 22), or to failure of melanogenesis 

 without death of the cell (albinism c a c a ). e54 - 1204 - 1315 



The primary differentiation in quality is between cells that produce eumelanin 

 (sepia, brown) and phaeomelanin (yellow). The most important factor in relation 

 to this differentiation in the guinea pig is the e locus (E typically self-eumelanic, ee 

 typicaly self-phaeomelanic, e p e p , e p e a mosaic (tortoise-shell) of the colors found with 

 E and e). The ^-alleles cannot be supposed to determine differentiation of melanocytes 

 directly but merely to produce an effect that predisposes toward one or the other type 

 of differentiation (at 5 and 6 in figure 22) . Thus with EA each hair follicle produces 

 (at 8 in figure 22) the same sort of yellow as with ee, during a brief phase in the growth 

 cycle of each hair, in spite of the presence of E. This results in a subterminal yellow 

 band in otherwise eumelanic hair. A locus with similar action in the mouse acts on 

 the melanocytes from adjacent epidermal cells. 1203 On the other hand, pigment cells 

 in animals with pure yellow coats at birth because of ee may produce much black 

 or brown pigment later (sootiness at 10 in figure 22) in the presence of favorable un- 

 analyzed heredity 2 {So) and low temperature. 1401 . Gene e p is definitely not mutable 

 in the germ line in inbred strains. The nature of the all-or-none process that occurs 

 early in development in tortoise-shells to distinguish different cell lineages (perhaps in 

 the epidermis) is not known. 



Cells with phaeomelanic differentiation produce only phaeomelanin but those 

 with eumelanic differentiation produce granules that, while largely eumelanic, may 

 contain a small amount of phaeomelanin, which is revealed under conditions that differ- 

 entially reduce the eumelanic constituent. 



Visual grades, based on standard squares of skin chosen so that each grade is barely 

 distinguished from the preceding, have been assigned each animal at birth and often 

 later. The corresponding amounts of pigment, relative to intense black (grade 21) 

 or intense yellow (grade 1 1 ) have been estimated by extraction from weighed samples 

 of hair and colorimetry. 542 ' 1096 - 1453 In the later years, 1415 reflectionmeter readings 

 (R) were taken of many genotypes using amber, green, and blue filters. Indices 

 closely paralleling the visual grades were obtained by the equation 



I x = \0{\ogR w -logR x ) 



