24 HEREDITY OF SKIN COLOR IN NEGRO-WHITE CROSSES. 



well to have made the limits of the first class 0-12 and of the second 

 13-25, thus equalizing them and making fuller allowance for the greater 

 range of o-pigmentation in transparent-skinned children than in their 

 thick-skinned adults. Otherwise agreement is fair, save for a deficiency 

 of children in the 4-f actor group, which is probably due to the fact 

 that some of the young children had not yet fully formed the dark 

 grades of pigment. On the whole, a comparison of the realized and 

 expected series gives conclusive testimony to the validity of the 

 hypothesis with which we started. There are two gametic (Jour somatic) 

 factors for black in negro skin pigmentation. 



VII. Is THERE A SEX-LINKAGE OR SEX-DIMORPHISM IN SKIN COLOR? 



It is well known that in the Silkie fowl abundant black pigment 

 is formed in the skin, so that these birds, though albinic in plumage, 

 are melanic sports in the formation of pigment in skin and connective 

 tissues. In such Silkie fowl, as Bateson and Punnett have shown, the 

 inhibition of pigment is sex-linked, appearing in the daughters of 

 hybrids between Silkies and clear-skinned fowl only when carried by 

 their fathers. That is, if the father is non-Silkie (i.e., non-pigmented) 

 while the mother is pigmented, the daughters (like the sons) are non- 

 pigmented; but if the father is Silkie (i.e., lacks the inhibitor for pig- 

 mentation) the daughters alone will lack it, and so the daughters will 

 have black skins. It is known that conditions are reversed in mam- 

 mals that sons take after their mothers. Is there any evidence of a 

 preponderance of light sons from matings of light mothers or of dark 

 sons from matings of dark mothers? 



I have tabulated the sexes of dark children of fathers with no 

 factor by mothers with 2 factors (table 10) and find 9 sons and 12 

 daughters ; also the sexes of the light children of a father with 2 factors 

 and mother with no factor (table 17) and find 7 sons and 7 daughters. 

 Indeed, the sons of light mothers are relatively as apt to be dark as 

 light ; and the sons of dark mothers are relatively as apt to be light as 

 dark. There is no sex-linkage in the inheritance of human skin color. 



Similarly, there is no evidence of sex-dimorphism in skin color. 

 Schiller-Tietz (1902) states that J. M. Hildenbrand finds the negro 

 women on the average somewhat lighter than the men and ascribes 

 this difference to the greater tanning of the skin of the males. Our 

 determinations were made on the untanned skin. I have separated 

 the sexes and calculated the average per cent of N in each. The adult 

 males of our data average N 23 per cent; the adult females N 24 per 

 cent. There is here no evidence of any difference of pigmentation in 

 the untanned skin of the two sexes. 



Though there is no evidence of a sex-dimorphism in adult skin 

 color there is a sex-difference in the rate of development of pigmenta- 

 tion. Thus, in children under one year the males have an average of 



