120 HEREDITY IN RELATION TO EUGENICS 



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Fig. 87. — Pedigree of night blindness in a negro family, many of whom 

 were personally examined by Dr. Bordley. IV, 18, 19, are doubtful. All 

 solid block symbols stand for affected persons; clear symbols unaffected. The 

 blindness is progressive and ends in death within 16 months after blindness 

 becomes complete. All affected persons have an affected parent. Night 

 blindness is a positive trait. Bordley, 1908. 



in females in order to develop; but in both Nettleship's and 

 Bordley's families even simplex females have night blind- 

 ness. Ordinarily, consequently^, while night blind people 

 should not reproduce, normal males from such stock may do 

 so with impunity, but normal females may have children 

 only when all their brothers (more than two) are without 

 the defect; for normal females, in most night blind families, 

 may carry the disease. 



n. Color Blindness. — The inability to distinguish certain 

 colors, notably red and green, is not a rare condition but 

 much less common in women than men (in Europe, 4 per 

 cent males, 0.5 per cent females) . The method of inheritance 

 of the condition is much the same as that of atrophy of the 

 optic nerve and night blindness; namely, that color blind 

 males do not have color blind sons but that females free 

 from color blindness may have sons with it (Fig. 88). 



The eugenic conclusion is that while color blind males 

 will have no color blind sons and, typically, no color blind 

 offspring of either sex yet their daughters, married to men 

 of normal stock, will have color blind sons. 



To the ordinary rule there are various exceptions. Daugh- 



