been demonstrated for the patterns 

 of piebald mice, hooded rats, and 

 Dutch rabbits, while a dominant fac- 

 tor is responsible for the English pat- 

 tern of rabbits and the white blaze in 



man. 



The mechanism by which such pat- 

 terns are determined is interesting to 

 speculate upon, but very little is vet 

 known. In the case of the English 

 rabbit, Onslow has demonstrated the 

 presence of an inhibitor which pre- 

 vents the oxidation of tyrosin bv tyro- 

 sinase. Apparently the power of a cell 

 to produce this inhibitor is determined 

 by a variety of conditions of which 

 the level of the English factor (absent, 

 heterozygous or homozygous) is one 

 and differences brought about in re- 

 gional differentiation another. As a re- 

 sult of the combined effects of these 

 conditions a given cell either has no 

 power to produce the inhibitor or can 

 produce sufficient to inhibit anv in- 

 tensitv of color. The extent of white 

 patterns seems to be the same in gen- 

 eral whether the ground color is 

 intense or dilute. In Dutch rabbits 

 Onslow found no enzyme inhibitor 

 but simply an absence of peroxidase. 

 Here we must suppose that some es- 

 sential condition in the cells for pro- 

 duction of enzyme I is determined bv 

 the array of recessive white pattern 

 factors in conjunction with regional 

 differentiation. 



The maltese type of dilution which 

 appears under the microscope in such 

 a case as the blue rabbit as due to an 

 alternation of colorless spaces with in- 

 tense pigmentation within each hair 

 is put provisionallv in class la. Reces- 

 sive factors which determine simul- 

 taneouslv maltese dilution of black and 

 the homologous kind of dilution of 

 vellow have been demonstrated in 

 dogs, cats, mice and rabbits. 



Class 1/7.— Correlated dilution of 

 black and yellow is probably very 



WRIGHT 



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 differen- 

 tiates dun and yellow cattle from 

 blacks and reds.^^ The sepia and yel- 

 low guinea-pigs differ from black and 

 red ones by a unit recessive factor.^^ 

 There are two more allelomorphs of 

 this dilution factor. In red-eved dilutes 

 vellow disappears entirelv, giving place 

 to white, and eye color becomes dis- 

 tinctly dilute while in the lowest re- 

 cessives of the series— the albinos- 

 sepia as well as yellow nearly disap- 

 pears and the eyes become pink. 



Rats show a series of three allelo- 

 morphs which seem to be comparable 

 to the guinea-pig albino series. ^^ The 

 lowest recessive determines complete 

 albinism while the second member of 

 the series is much like red-eyed dilu- 

 tion in guinea-pigs. Black is diluted, 

 vellow is reduced to white and the eve 

 color to red. Rabbits show a series of 

 three allelomorphs in the complete 

 albino, Himalavan albino and fullv 

 colored varieties. Albinism is found in 

 manv other mammals and alwavs 

 seems to be recessive in inheritance. 

 There are a number of curious fea- 

 tures in the albino series in guinea-pigs 

 which have had much to do with 

 shaping the hypothesis advanced here. 

 The table below shows roughlv the 

 model grades of intensity^ of vellow 

 and black fur and eve color found 

 with each of the ten possible zvgotic 

 formulae. Guinea-pigs of these ten 

 formulae can easily^ be distinguished 

 bv the results of crosses with albinos, 

 the lowest recessives. The full evi- 



17 Wilson, J., 1909. Sci. Proc. Roy. Dub. 

 Soc, 12:66. 



isCasde, W. E., and S. Wright, 1916. 

 Cam. hist. Wash. Publ., 241. 



19 Whiting, P., 1916. Set., n.s., 43:781. 



