Wright: Color Inheritance in Mammals 



229 



the usual white color. The iris is 

 usually pale blue rather than colorless. 

 The skin may show some pigmentation, 

 especially as freckles. In the colored 

 races such freckles are the rule. Even 

 in these races, the gap between normals 

 and albinos is bridged by various dilute 

 types. There are all grades of imper- 

 fect albinos which may or may not 

 show visual difficulties. 



It is evident that no one unit factor 

 can explain all the phenomena of albi- 

 nism. Nevertheless, the usual mode of 

 inheritance within families is clear. We 

 do not find long unbroken lines of de- 

 scent as in premature grayness and 

 white spotting. In a typical pedigree, 

 albinism will be found, perhaps, in sev- 

 eral children of a family. The parents 

 may both be normal, but the trait will 

 be found cropping out in several more 

 or less distant cousins. One of the very 

 striking features of pedigrees, as Pear- 

 son points out, is the great frequency 

 of cousin marriages among the parents 

 of albinos. Castle^ and Farabee, as 

 early as 1903, pointed out the proba- 

 bility that albinism behaved as a reces- 

 sive unit, and Davenport* demonstrated 

 it in a considerable number of pedi- 

 grees, including three cases in which 

 albino by albino had had only albino 

 children (fovuMnall). Apert^ analyzed 

 the data of Pearson, Nettleship, and 

 Usher and pointed out that the ratios 

 of albino to normals, correctly inter- 

 preted, was close to that expected on 

 the hypothesis of a recessive unit. 



The view that albinism is, in general, 

 due to recessive factors may safely be 

 accepted, but it must be pointed out, 

 as in the case of piebalds, that different 

 factors may be involved in different 

 families. The different grades may be 

 due to multiple allelomorphs as found 

 in albino rabbits, guinea-pigs, and rats 

 or may be due to combinations of in- 

 dependent factors or both. It is easy 

 to see that a dilution factor which 

 would produce very little visible effect 

 in the negro race might be enough to 

 reduce a blond European to an albino 



with imperfect vision and nystagmus. 

 Indeed blondness itself may be looked 

 upon as a grade of imperfect albinism. 

 Another problem is a curious relation 

 of albinism to red hair. Davenport 

 noted that among forty-two parents of 

 albinos in his tables, eleven had some 

 grade of red hair. This is above the 

 usual per cent of red hair in the popu- 

 lation involved. About 4% of the popu- 

 lation in England, Scotland, or Ireland 

 have distinctly red hair, according to 

 Pearson. The number is larger if we 

 include chestnut, auburn, and yellow. 

 Data collected by Holmes and Loomis^° 

 at the University of Wisconsin contain 

 8.8%, including these colors. Pearson 

 noted a similar excess of red among 

 the relatives of albinos in the very ex- 

 tensive data referred to; 13.0% of the 

 parents, 12.9% of the siblings, and 

 8.6% of the uncles and aunts showed 

 some trace of red. Pearson also em- 

 phasizes the point that albinos may 

 have yellow or even light red hair 

 themselves. 



These facts seem, at first, very diffi- 

 cult to explain when we recall that in 

 rabbits, guinea-pigs, rats, and dogs, 

 red is very much more quickly reduced 

 to white than is black by grades of im- 

 perfect albinism. The subject will be 

 discussed further when the data on the 

 inheritance of red hair are examined. 

 Here it is sufficient to note that albinism 

 cannot always be a single unit. We 

 may say that albinism in most families 

 is due to recessive unit factors, but we 

 must be cautious in identifying the al- 

 binism of different families with each 

 other and with the albinism of lower 

 animals. 



THE COMMON VARIATIONS IN COLOR 



Of more interest than the heredity 

 of these unusual traits is that of the 

 common variations in skin, hair, and 

 eye color. Unfortunately the problems 

 are much more difficult. Wlien even 

 such an uncommon color as red hair 

 occurs in two relatives one can have no 

 such assurance that the same factor is 



^Castle, W. E., and W. E. Farabee, 1903, Sci. N. S., 17. 

 'Davenport, C. B., and G. C. Davenport, 1910, Amcr. Nat.. 44:705-731. 

 7-221-223 



'Apert, 1914, Jour. Her.. 5:492-497. 



'"Holmes, S. J., and H. M. Loomis, 1910, Biol. Bull., 18:50-65. 



1916, Jour. Her. 



