GENIC INTERACTION 171 



attributed to color factors (E, e; A, a; C, c k , c d , c r , c a ; P, p; F, f), but the correlation 

 with the intensity of pigment production is far from perfect. The dopa reaction has 

 been studied in frozen sections of the skin at birth 735 ' 113 ° and in colorless extracts 

 from the skin at birth. 433 Foster 394 has studied both the oxygen consumption curves 

 and amount of darkening in the Warburg apparatus on adding tyrosine or dopa to 

 homogenates of fetal skin. 



There are some apparent inconsistencies in the results, but these seem to trace 

 largely to the stage of the reaction on which observation was focussed. Foster's tech- 

 nique was designed to distinguish differences in moderate to strong reactions and gave 

 less certain results in distinguishing very feeble reactions from the background oxygen 

 consumption of controls. The earlier studies of the dopa reaction distinguished 

 different end results of very feeble reactions from that in controls but gave such saturated 

 results from only moderately strong reactions that these were not always distinguished 

 from very strong ones. The dopa reactions in frozen sections were moreover very 

 seriously obscured where there was much natural pigment (dark sepias, dark browns). 



It may be noted first that tyrosinase and dopa oxidase activities were very greatly 

 reduced by replacing E by ^. 394 Foster found similar reduction in the yellow phase of 

 agouti follicles (EA) in comparison with the black phase (Eaa) . Much black melanin 

 was, however, ultimately produced from dopa in frozen sections 735 - 1130 and extracts 433 

 from intense yellow skin although both Russell and Ginsburg found significantly less 

 than from even pale sepia. 



Foster was unable to demonstrate any tyrosinase or dopa-oxidase activity from 

 lower c compounds including EBc k c T PPFF which is intense black. Russell found 

 considerable ultimate darkening of dopa in frozen sections from pale sepias (EBppFF) 

 and yellows (eeFF) carrying c k c k , c k c d or c d c d though much less than with C, and even 

 some darkening in the corresponding heterozygotes c kd c ra . Russell, Ginsburg, and 

 Foster all found no evidence of a reaction from the near blacks of genotype c r c r . It 

 would appear that the enzymes produced by the c compounds tend to be lost, after 

 forming the natural pigments, almost as rapidly as they are produced unless produced 

 in great excess by C. We have already noted the thermolability of the product of the 

 lower c alleles which would especially affect Foster's studies of homogenates from fetal 

 skin. 



Foster found that homogenates from pale sepias (EBCppF) and pale browns 

 (EbbCppF) oxidized tyrosine or dopa significantly less rapidly than homogenates from 

 intense blacks or browns (P in place of pp) but the reduction was far from proportional 

 to the great reduction in amount of natural pigment (in contrast with his results from 

 lower c compounds) . This agreed with Kroning's results in hair 735 and Russell's results 

 at least in the epidermis. Replacement of F by /caused no reduction in tyrosine or 

 dopa oxidase activity in intense blacks or browns. With the same replacement in the 

 presence of pp, the moderately strong reactions were reduced to zero (tyrosinase) or 

 very weak (dopa oxidase) according to Foster. These parallel the striking interaction 

 effect in vivo {EPF and EPff, equally intense; EppF, pale; Eppff, very pale). The dopa 



