168 PHYSIOLOGIC GENETICS 



probably Cc x Ppff in which x is any lower c allele. Thus in the presence of Ebb, 

 c x c z ppjy is pure white, Cppff has a trace of color, c x c x ppF is very pale brown, CppF pale 

 brown, c x c a Pff slightly dilute brown, c k c k Pff intense brown, CPpff possibly slightly 

 more intense, but CPpFF slightly dilute (dingy on the head), CPPff slightly more dilute 

 and CCPPFF often with less than half as much pigment as the intense browns. Re- 

 placement of CC by Cc x in these has a slight darkening effect demonstrated only in 

 adults. The type of dilution in the higher combinations is of a peculiar sort (dinginess) 

 which ranges from a mere sprinkling of dark tipped light hairs on the cheeks and nape 

 to uniform dilution of all hairs, except at the extreme tips, to a color as pale as pale 

 sepia (only in EbbCCPPFF with favorable modifiers). This type of dilution could 

 not be produced at all in the presence of c k c k or oi pp. This effect is represented in 

 figure 22 as the result of a destructive action of CPF product in excess of that necessary 

 to saturate the limited bb product. The failure to observe any such effect in blacks 

 in this colony may be interpreted as due to the higher ceiling provided by the B product. 

 Ibsen and Goertzen 624 described an incompletely dominant modifier of dinginess [W) 

 which completely inhibits brown pigment in a subterminal band in EbbCCPPFF. 

 They found a slight effect in blacks in the presence of WW. While W was clearly 

 absent from my colony, dinginess was much affected by other modifiers. It was possible 

 to bring even EbbCCPPFF to full intensity in a few cases by selection. 



We will go back to consider modifiers of some of the other processes here. The 

 most important genetic ones are Dm, dm and Si, si (figure 24). 1415, 1432, 1440 The 

 most conspicuous effect of sisi in otherwise intense animals is to cause a sprinkling of 

 white hairs (silvering) in the coat which does not progress after birth in contrast with 

 the effect of grgr (grizzling) 7 * 5 - 1431 with no effect at birth but progressive whitening 

 later. We are here concerned with dilution effects of si on colored hairs. Replacement 

 of Dm by dm (diminution) has no recognizable effect on intense (C) blacks or yellows but 

 causes dilution in lower c compounds, most conspicuously in c d c a (both sepia and yellow) 

 and in c r c a sepias. Factors si and dm dilute both colors cumulatively (figure 24). 

 The combination sisidmdm is pure white except for occasional pale spots on the head. 

 Eye color is slightly reduced. There are other effects which suggest that the effect 

 of these genes is on the vitality and metabolic efficiency of certain types of cells rather 

 than on the pigment process in as specific a way as seems to be the case with many of 

 the other loci. Animals with sisidmdm suffer a high mortality after birth and an 

 anemia in which the red blood count is reduced by a third. In males, testicular size is 

 .reduced to 25 per cent of normal and there is complete absence of spermatogenesis 

 and thus sterility. About half the females tested have been sterile and the remainder 

 rather low in productivity. Assuming that the effect of these genes on color is on meta- 

 bolic efficiency of the pigment cells (if not actually lethal to these as in silvered hairs) 

 it appears that among the specific color factors only the efficiency of the c alleles is 

 affected. The effects of si and dm have been studied intensively only in dark sepias 

 EBP and yellows eeFF, but as far as determined they have proportional effects in 

 browns (EbbP), pale sepias (EBppF), and pale yellows (eeff). 



