DISTRIBUTIVE COLOR FACTORS. 



31 



never been found to transmit the agouti pattern, it seems that Morgan has been 

 misled by the superficial appearance of "ticking" in the hair. 



That superficial ticking is not identical with the agouti pattern is observ- 

 able in the case of "sable" mice, as will be more fully discussed later. It is also 

 observable in certain "dingy" or "smoky" young produced by Himalayan 

 rabbits. In this case there may be as many as two light bands in the " ticked " 

 hairs, the dark portions of which seem to contain a small amount of black pig- 

 ment producing the dingy appearance alluded to. In such cases it is known 

 that neither parent carries the agouti pattern, yet here are hairs with an apj)a- 

 rently similar ticking; therefore not all ticking of rodent hairs is due to the 

 same factor. 



We may now consider crosses in which the "agouti" pattern is involved. 

 In the cross between homozygous agouti and non-agouti forms, Fi consists 

 entirely of agouti (black agouti or brown agouti) animals, 344 young having 

 been thus obtained. When certain of these heterozygous F\ animals were 

 crossed inter se there were obtained : 



If the heterozygous Fi agoutis are crossed with non-agouti mice we should 

 expect equality of agouti and non-agouti young. In this cross I have obtained : 



When non-agouti animals are crossed inter se only non-agouti young are 

 produced, the numbers in this case being 1,812. 



From the result of these crosses it may be stated that the agouti pattern, 

 though it may or may not be a single unit character, certainly behaves like one. 

 Until it has been shown to be complex, the only safe course seems to be to con- 

 sider it simple and allelomorphic to its absence. 



7. The Distributive Factor, R. 



This factor produces the true yellow varieties of mice by an almost com- 

 plete exclusion of brown and black from the skin and hair. In those localities, 

 therefore, the process of pigment formation is carried only as far as the "yellow" 

 stage. The chief point of importance in connection with the appearance of 

 yellow in the coat is this: Yellow-pigment granules, so far as experimental evi- 

 dence goes, never appear in the skin and hair of mice unless certain distributive 

 factors (A or R) are present to exclude part or all of the brown and black from 

 those localities. In such regions oxidation beyond the yellow stage is impos- 



