276 Heredity and Eugenics 



partially or fully prevents the development of the tooth 

 germ. 



Some peculiarities of the eye are clearly inherited. 

 Thus the condition in which the lens of the eye becomes 

 clouded and opaque seems to be due to a positive determiner, 

 and this is most clearly seen when cataract appears in 

 middle life. For, all cases of presenile cataract appear in the 

 offspring of affected persons, and two unaffected persons 

 probably never have descendants with cataract (Fig. 84). 



6^ 



FIG. 84. Pedigree of a family with presenile cataract (black symbols). 

 Numbers in circles indicate unaffected individuals. Cataract in offspring of 

 affected parents only. 



The same law holds for a peculiarity of the eye such that 

 the affected person cannot see by weak light, such as the 

 light of lamp, electric lights, etc. (night blindness). There 

 is clearly a positive determiner, since affected persons have 

 affected offspring, but if the parents are not affected (thus 

 proving the absence of the determiner from their germ 

 plasm), the offspring are never affected. These con- 

 clusions are based on the remarkable pedigree compiled 

 by Nettleship, involving over two thousand individuals. 



