Wright: Color Inheritance in Mammals 



377 



so-called partial albinism of piebalds. 

 In mice, no intermediate allelomorphs 

 have been discovered, but it is most 

 probable that it is homologous with 

 albinism in the other rodents and there- 

 fore to be placed in class lb. 



VARIATIONS OF YELLOW 



When Cuenot'-- discovered that the 

 differences between the various colors 

 could be transmitted by albinos as 

 well as by mice in which the colors were 

 visible he devised the very successful 

 factor hypothesis as an explanation. 

 He did not, however, attempt to force 

 all color differences into pairs of allelo- 

 morphs. He found that yellow, gray 

 and black behaved as if due to three 

 variations of the same factor, each one 

 being an allelomorph of either of the 

 others. Later-^ he added light-bellied 

 agouti as a variation between yellow 

 and ticked-bellied agouti or gray. 

 Cuenot^"* also discovered the curious 

 fact that yellows could not be obtained 

 in a homozygous condition, a result 

 confirmed by Miss Durham, ^^ Castle, 

 Little'^^ and others. Kirkham's^^ recent 

 demonstration of excessive degeneration 

 among embryos from the cross yellow by 

 yellow is a most interesting physical 

 confirmation of a conclusion based on 

 genetic evidence. 



It may be noted here that other kinds 

 of yellows may exist among mice. 

 Hagedoorn-^ has announced some very 

 peculiar results including work on 

 dominant and recessive strains of yellow 

 mice both of which could be bred true 

 and, therefore, were different from the 

 yellows of all other investigators. His 

 results, however, need confirmation as 

 they were in many cases inconsistent 

 with his own interpretation.-^ 



A white-bellied gray mouse (AlAJ 

 differs from a black (AbAb") in several 



respects. Each hair on the back is 

 black at the base and tip with a yellow 

 band between and on the belly each 

 hair may be white throughout. A 

 black mouse has solid black hair all 

 over. Thus the same factor appears 

 to determine a pattern dift"erence be- 

 tween back and belly, a curious cycle 

 in the development of colors on the back 

 and complete or nearly complete inhibi- 

 tion of all color on the belly. Physio- 

 logically, however, the case is probably 

 not as complex as at first appears. By 

 crossing with very intensely colored 

 mice the belly may be changed from 

 white to yellow, and black can be 

 brought out at the base of the hairs. 

 It is evident that the belly really differs 

 from the back in having a widening of 

 the yellow band to cover the whole hair 

 and in a dilution of this yellow nearly or 

 entirely to white. Neither of these 

 differences is necessarily determined by 

 the agouti factor, but is merely revealed 

 by the latter. The case is entirely 

 comparable with that of the gray rabbit 

 discussed in the previous paper. No 

 doubt the belly differs from the back 

 even in solid blacks in having a lower 

 density of black and also a lower level of 

 color production in general, only re- 

 vealed when the agouti factor introduces 

 an inhibitor of black into all parts of 

 the coat. This inhibitor finds less of 

 the black-producing enzyme to inhibit 

 on the belly, and so produces a wider 

 band which, however, appears white 

 instead of yellow because differentiation 

 during development has left the general 

 level of color production on the belly 

 below the threshold for yellow. Thus 

 the four allelomorphs of the agouti 

 series can be interpreted as determining 

 simply four different rates of production 

 of an inhibitor for black. 



A series of variations of vellow has 



22 Cuenot, L. 1903. Ibid. (A), 1, Notes et revue, 33-41. 



23 Cuenot, L. 1907. Loc. cit. 



24 Cuenot, L. 1905, 1908, 1911. Loc. cit. 



25 Durham, F. M. 1911. Jour. Gen., 1:159-178. 



28 Castle, W. E. 1906. Sci. N. S., 24:275-280. (And C. C. Little. 1910. Sci. N. S., 

 32:868-870.) 



2" Kirkham, W. B. 1916. Abstracts Amer. Soc. ZooL. p. 8. 



2SHagedoorn, A. 1908, 1909. Univ. Cat. Pub. in Physiol. 1912. Zeit. f. Abst. Ver., 

 6:97-136. 



23 See for example, Cuenot, 1911, loc. cit.; Plate, 1912, Zeit. f. Abst. Ver., 6; Little, 1914, 

 48:74-82; Lang, 1914, Ex per intent ell e Vererbungslehre. 



