GENIC INTERACTION 167 



the full amount possible with each c compound. Compounds FF, Ff or ff make no 

 detectable difference in the intensities of any of these. The replacement of P in 

 EBPF by pp (figure 23) results in a great reduction in the amount of pigment in both 

 coat and eyes (pink). There is disproportionately great reduction in the lower c 

 compounds, especially c r c r and c r c a . With c r c a the color is often indistinguishable from 

 white at birth, indicating a low threshold (at 24 in figure 22). The fact that even 

 with c r c r there is less pigment than with c d c a , in contrast with the situation in the presence 

 of P, seems to require a carrying through of the effect of the specificity difference between 

 c r and c d to the reaction at 22 (in figure 22). 



The replacement of F in EBppF by ff reduces pale sepia to a very much paler 

 brownish cream, sometimes indistinguishable from the pure pale yellow of eec d c d ff. 

 This color is interpreted as due largely to the uncovering of a feeble underlying yellow 

 (at 1 5) in the absence of both P and F. The trace of eumelanin, usually present, is 

 attributable to feeble action of/ (at 23) . The fact that replacement of F by/f makes no 

 recognizable difference if P is present, but almost complete absence of eumelanin with 

 pp is interpreted as meaning that P, without F, is sufficiently efficient to produce as 

 much eumelanin as the c compound permits but that in the absence of P, F acts as a 

 feeble substitute, and if this also fails, only a trace at most can be produced by/. It 

 may be noted from figure 23 that with any lower c compound and Eppff the color 

 is pure white, indicating that the eumelanic and phaeomelanic processes fall below the 

 threshold in these cases. 



The replacement of B in EBP by bb gives brown in coat, skin, and eyes in place of 

 sepia and approximately halves the apparent intensity by colorimetric determinations 

 of these closely similar pigments. There is parallelism between sepia and brown 

 among the c compounds (figure 23) to the extent that c a c a is white and c d c a and c r c a 

 definitely more dilute than the others (except certain combinations involvoing C), 

 but the differences among all the other c compounds are much less in browns not only 

 absolutely but on a percentage basis. This can be interpreted on the hypothesis that 

 bb imposes a ceiling on the possible amount of pigment at about half that imposed by 

 B and that this ceiling is approached to such an extent even with c r c a and c d c a that there 

 is no possibility of much further increase with higher compounds. 



If P in EbbPF is replaced by pp, there is marked reduction in intensity but not as 

 much proportionately as in the sepias. The eyes are again pink. There is a marked 

 qualitative difference from the pale sepias (figure 26) in contrast with the slight apparent 

 difference between dark browns and dark sepias of the same intensity. Pale browns 

 with C have much more than half as much pigment as the corresponding pale sepias 

 and among the lower c compounds there are no consistent differences in quantity in the 

 somewhat unsatisfactory data on this point. The smallness of the quantitative differ- 

 ence in comparison with that in the P eumelanics may be interpreted as due to remote- 

 ness from the ceiling. Ebbppff is wholly indistinguishable from EBppff 



The most remarkable interaction effect is one that affects only browns of genotype 

 EbbCP. li31 - 1439 There is an optimum genotype with respect to the c, p, and /loci, 



