14 HEREDITY OF SKIN COLOR IN NEGRO-WHITE CROSSES. 



the evidence of its compoundness. Now the Bermuda polygon (fig. 3) 

 gives evidence of five maxima, as follows: (i) about 5 per cent; (2) 

 about 18 per cent; (3) about 35 per cent; (4) about 46 per cent; and 

 (5) about 70 per cent. Smoothing, somewhat, the empirical results, we 

 may take the probable range of effect of our five hypothetical factors 

 as given in table 7. 



Table 7. — Classification of hybrid skin colors on the basis of the factor hypothesis* 



We are now in a position to test this hypothesis in various matings, 

 of which the ancestry is not precisely known. We may assume, what 

 is close to the truth, that parents with a skin color of N 9 or less have 

 the gametic formula 0262;* those with N between the grades of 10 

 and 25, inclusive, have the formula Aabi; those with N between the 

 grades of 26 and 40, AaBb; those with N between the grades 41 and 

 55, A2Bb; those with N between the grades 56 and 78, A2B2. We can 

 calculate the proportion of offspring of each zygotic composition and 

 compare with the proportion of offspring of each class of color. If the 

 agreement is close, the hypothesis is justified; otherwise it is not 

 justified. 



A word about the determination of the class ranges. It was first 

 determined empirically that grades below 10 per cent were not only 

 common among Caucasians, but parents with grades less than 10 per 

 cent do not, with rare exceptions, have children of darker skin color 

 than themselves. It was then decided to divide the whole range be- 

 tween 10 and 70 into four equal classes with a range of 15 points each. 

 In the final adjustment the first of these classes contains 16 points and 

 the last was extended to 78 to include a few very dark individuals 

 found in Bermuda. There is reason for thinking that the range of the 

 first two classes should be somewhat equalized in the offspring. For 

 "offspring" are, on the whole, younger than parents, and their skin 

 has undergone less of that fading which is found in older persons. A 

 filial grade of 10 or 11 corresponds to a parental grade of 9. Accord- 

 ingly, the limits for the two lower grades of skin color are set, in the 

 offspring, at o-ii and 12-25, respectively. 



•Capital letters {A, B) indicate presence of the factor, lower case letters {a, b) 

 absence of the factor. 



