232 



The Journal of Heredity 



ously maltesc dilution of black and the 

 homologous kind of dilution of yellow 

 have been demonstrated in do^^^s, cats, 

 mice and rabbits. 



Class \b. — Correlated dilution of l)lack 

 and yellow is jjrobably very common in 

 mammals. A case which seems to be 

 dominant is found in the factor by 

 which dun, mouse and cream-colored 

 horses differ from bays, blacks and 

 chestnuts respectively. An imperfectly 

 dominant factor differentiates dun and 

 yellow cattle from blacks and reds.^^ 

 The sepia and yellow j^uinea-pigs differ 

 from black and red ones by a unit 

 recessive factor. ^^ There are two more 

 allelomor]:)hs of this dilution factor. 

 In red-eyed dilutes yellow disa]j]x^ars 

 entirely, giving ]jlace to white, and eye 

 color becomes distinctly dilute while 

 in the lowest recessives of the series — 

 the albinos — sej^ia as well as yellow 

 nearly disappears and the eyes become 

 pink. 



Rats show a series of three allelo- 

 morphs which seem to be comjjarable to 

 the guinea-pig albino series.'" The 

 lowest recessive determines comjjlete 

 albinism while the second member of the 

 series is much like red-eyed dilution in 

 guinea-pigs. Black is diluted, yellow 

 is reduced to white and the eye color 

 to red. Rabbits show a series of three 

 allelomorphs in the complete albino, 

 Himalayan albino and fully colored 

 varieties. AlV)inism is found in many 

 other mammals and always seems to be 

 recessive in inheritance. There are a 

 number of curious features in the albino 

 series in guinea-])igs which have had 

 much to do with shaj^ing the hypothesis 

 advanced here. The table below shows 

 roughly the model grades of inten- 

 sity of yellow and black fur and eye 

 color found with each of the ten i)os- 

 sible zygotic formulae. Guinea-i)igs of 

 these ten formulae can easily be dis- 

 tinguishvd by the results of crosses 

 with albinos, the lowest recessives. 

 The full evidence has been given in a 

 recent jniblication.^" 



CC 

 CCd 



CCr 



CCa 



CdCd 



CdCr 



CdCa 



CrCr 



CrCa 



CaCa 



Yellow fur 

 Red 

 Red 

 Red 

 Red 

 Yellow 

 Cream 

 Cream 

 White 

 White 

 White 



Black fur 



Black 



Black 



Black 



J^lack 



1 )ark scijia 



Dark sepia 



]>ight sepia 



Dark sejiia 



Light sej)ia 



Sooty white 



Black eye 

 Black 

 Black 

 Black 

 Black 

 Black 

 Black 

 Black 

 Red 

 Red 

 Pink 



There are a number of ways in which 

 such a series of four allelomorphs as C, 

 C^, Cr and Ca can be interpreted. 

 Comi)lete linkage could ex])lain the 

 mere ratios obtained in crosses but leaves 

 wholly unexplained the graded series of 

 l)hysiological effects. Four diverse non- 

 linear variations of a factor would be 

 another jjossibility. It seems most in 

 harmony with the broad facts of the 

 series, however, to consider these as 

 four levels in potency of some one 

 factor. But with this interpretation 

 two peculiarities of the series stand out 

 at once. First, we get complete albin- 

 ism in yellow parts of the fur at a level 

 in the scries (CrCr) at which black in 

 fur and eye is still quite intense. This 

 renders it unlikely that black results 

 from the further oxidation of yellow 

 ]3igment or the reverse. The hypothesis 

 suggested is that the efficiency of enzyme 

 I, the basic enzyme for color ]5roduction 

 but which ])roduces only yellow when 

 acting alone, is increased by union 

 with enzyme II so that black ])igment 

 is produced at a much lower threshold 

 than yellow. The second curious fact 

 is the bimodal curve of intensity in 

 black fur in ]jassing down the series of 

 zygotic formulae. If Cd Ca determines 

 a higher potency of enzx-me I than 

 CfCr. it wotild seem that it should 

 determine a higher intensity of color 

 everywhere. Yet CdCa gives a dis- 

 tinctly ])aler black than CrCr 1)tU a more 

 intense yellow and eye color. It will 

 be noticed that this irregularity occurs 

 just at the jioint at which \ellow is able 

 toa]Ji)ear and the explanation suggested 

 is that competition of enz\^^le I with the 



•^Wilson, T., 1909. Sci. Proc. Row Pub. Soc, 12: 66. 

 '"Castle, W. E., aild S. Wright, 1916. Carn. Inst. Wash. I'ul. 

 '"Whiting. P.. 1916. Sci. N. S., 4.S: 7S1. 

 "Castle, W. E., and S. Wright, Inc. cil. 



241. 



