XII] 



Cataract 



217 



Cataract. A study of the extensive collection of 

 evidence made by Nettleship shows that several forms of 

 cataract are usually transmitted as dominants, the descent 

 passing through the affected persons. This is shown most 

 clearly in regard to the congenital varieties. In these the 

 cataract is present and generally recognized very early in 

 life. Some of the family records exemplifying descents of 

 this kind are very extensive. Occasionally there is descent 



ffitis 



Fig. 30. Descent of prae-senile cataract. The condition was here 

 transmitted, exceptionally, through one individual who was probably 

 unaffected. (Condensed from Nettleship's chart.) 



of the cataract through a parent recorded as unaffected, but 

 the course of the transmission is usually normal. Whether 

 in the exceptional cases the apparently normal persons who 

 transmitted were in reality slightly affected cannot yet be 

 said, but it is noticeable that several of these alleged excep- 

 tions occur in the case of progenitors whose state is only 

 known by tradition (Fig. 30). 



Cataracts acquired later in life, and even the senile forms, 

 seem, so far as the evidence goes, to follow the same rule in 



