INHERITANCE OF HUMAN CHARACTERS 



461 



matings by = . The circles are of solid color in individuals affected 

 with the deformity, open O in normal individuals. The character 

 seems to behave like a Mendelian dominant, though one could make 

 no very positive assertion on this point from so few individuals. But 

 it is very evident that such a physical character once in the stock is 

 transmitted generation after generation, reappearing continually in 

 the offspring. 



Below there is presented a chart (Fig. 97) of the transmission 

 of cataract. This disease is characterized by the appearance of 

 an opaque area in the usually transparent parts of the eye, 







99 f xd* x 9 



C3) (?i) 



f t 





Df*o i& 



t ? 



L 



@ O O 99 

 O 9 



FIG. 97. Inheritance of one form of cataract. Modified from Nettleship's 

 chart. The diagram reads thus: A man with cataract married a normal woman; 

 of their eight children six were affected with the disease. One of these married 

 an unaffected man; three of the children of this union were normal, sex unrecorded, 

 two defective. This same man married a second wife who was normal; their 

 eight children were all unaffected. So continue reading through five generations. 

 (From Downing.) 



ultimately rendering the person blind. In the particular form 

 of the disease here considered it does not develop until middle 

 life. Clarence Loeb in a study of hereditary blindness published 

 in 1909 tabulated the results of a study of 304 families in which 

 such blindness occurs. There were 1,012 children, 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. Similar extensive studies of congenital deafness 

 and deaf-mutism show that these are similarly heritable, though just 

 how the character behaves is not yet known, for undoubtedly under 

 "deafness" are included a variety of diseased conditions that must be 



