HUMAN HEREDITY AS REVEALED BY PEDIGREES 453 



Clarence Loeb in a study of hereditary blindness tabulated the results 

 of 304 families in which such blindness occurs. There were 1,012 chil- 

 dren, of whom 58 per cent were afflicted, which is about the percentage 

 expected when hybrid defectives mate with normal individuals and the 

 defect is a dominant character." 



A typical pedigree of a recessive character. — -A good example of 

 the mode of inheritance of an uncommon recessive character is shown 

 in Figure 83, the pedigree of a family line showing outcropping of 

 albinism, lack of pigment in hair, skin, and eyes. It is noteworthy 

 that the appearance of the character is relatively rare, only 19 out of 

 158 individuals in the pedigree being albinos or partial albinos; that 

 albino offspring are produced by two phenotypically normal (pig- 

 mented) parents; and that in generation VII, where several of the chil- 

 dren are albinos, the two parents are descended from common ances- 

 tors a few generations back, and are undoubtedly both heterozygous. 

 In this pedigree there are no cases of the mating of two albinos. If 

 such matings had occurred, the expectation, of course, would be that 

 all offspring would be albinos. Since we shall have to present further 

 examples of pedigrees of recessive characters in connection with the 

 account of the heredity of mental characters, we need give no further 

 examples here. 



It is also unnecessary to repeat in this place the description of such 

 sex-linked human characters as color-blindness, haemophilia, etc. 

 The accompanying chart (Fig. 84), which represents an investigation 

 of a Texas family connection made by the writer in 1910, is typical 

 for sex-linked characters. The existence of sex-linked heredity in man 

 goes far to support the contention of eugenists that heredity in man 

 follows the same rules as have been found to hold for the lower animals. 



As an example of human characters probably determined by cumu- 

 lative factors, we may cite the case of skin color. Whole races of man 

 are dark-skinned, others light-skinned. Some idea as to the mode of 

 heredity of skin color is obtained by studies of crosses between negro 

 and white individuals. "In a cross between a negro and a white 

 person," says Castle, "children are produced that are of an intermedi- 

 ate, but frequently variable skin color, and are known as mulattoes. 

 Mulattoes mating inter se produce an F 2 generation of highly variable 

 skin-color but rarely pure white. Davenport has concluded that two 

 independent Mendelian factors affecting skin-color are involved. This 

 explanation would lead us to expect one in sixteen of the F 2 mulatto 

 offspring to have skin as white as an European, even though his negro 



