THE FACTOR HYPOTHESIS 161 



Castle and other experimental breeders, as well as many 

 of the albino types that would double this list if c, or 

 the gene for absence of color, should be substituted for 

 C, the presence of color, in column 4 of the table on 

 page 160. 



Explanation of Symbols in the Foregoing Table 



Br = a gene acting on C to produce brown pigmentation. 



B = a gene acting on C to produce black pigmentation. 



Y = a gene acting on C to produce yellow pigmentation. 



The three genes, Y, B, Br, are present in every rabbit 

 gamete and up to date have not been separable as inde- 

 pendent unit characters, although they ha ve been sepa- 

 rated out in guinea-pigs and mice. There are no brown 

 rabbits, because black always goes linked with brown 

 covering the brown factor. Yellow rabbits result, as 

 explained below, through the action of factor e. 



C = a common color gene necessary for the production of any 

 pigment. It was discovered in 1903 by Cu6not. 



c = the absence of C which results in albinos, regardless of 

 whaterer pigment gene may be present. By changing C 

 to c, sixteen kinds of albinos would be added to this 

 catalogue, an addition of one phenotype and sixteen 

 genotypes, all looking alike but breeding differently. 



E = a gene governing the extension of black and brown pig- 

 ment, but not of yellow. 



e = the absence of extension or restriction of black and brown 

 pigment to the eyes and the skin of the extremities only, 

 while yellow remains extended and visible. Demonstrated 

 by Castle in 1909. 



7 = an intensity gene which determines the degree of pigmenta- 

 tion. It can be transmitted independently of C through 

 an albino. Discovered by Bateson and Durham in 1906. 



i = the absence of intensity or dilution. Dilute black = blue. 

 Dilute yellow = cream. Dilute gray = blue-gray. 



U = a gene for uniformity of pigmentation or "self-color" dis- 

 covered by Cuenot in 1904. 



u = the absence of uniformity which results in spotting with 



white. 



A = a pattern gene for agouti, or wild gray color, which causes 

 the brown and black pigments to be excluded from cer- 

 tain portions of each hair, resulting in the gray coat. 

 When present in the rabbit, it is also associated with 

 white or lighter color on the under surfaces of the tail 

 and belly. It was demonstrated by Castle in 1907. 



a = the absence of the agouti or pattern gene. 



