THE INHERITANCE OF FAMILY TRAITS 111 



normal daughter may transmit the defect to her sons. But 

 such a woman may marry with impunity if all of her brothers 

 are without defect and there are more than two of them. A 

 defective male should abstain from having children, for 

 some of his sons, at least, will probably be defective. 



d. Cataract. — This is an opacity of the lens which may 

 result from abnormal conditions originating in other parts 

 of the eye or body or they may seenmngly originate inside 

 the lens itself, in which case their heredity is marked. Prob- 



FiG. 77. — Ideal scheme showing inheritance of atrophy of the optic nerve. 

 The solid black squares indicate affected males; the heavy rings represent 

 non-affected females with defective germ cells. 



ably more pedigrees of cataract have been published than of 

 any other eye defect. Loeb (1909) refers to 304 famiUes of 

 which accounts have been printed. Of the 1012 children 

 in these pedigrees, 589 were affected, or 58 per cent.^ 



The usual method of inheritance is that of a positive 

 character. Affected individuals have either half or all of 

 their offspring affected, while two imaffected parents will 

 probably not have defective offpsring. However, as cataract 

 usually appears late in life it is not always possible to predict 

 whether the parent will become affected or not (Fig. 78). 



The eugenic rule is this: — If either parent has cataract 



at least half of the offspring will have it also. If a person 



belongs to a strain that has cataract but is free from it, 



advice must depend on the nature of the cataract. If in 



^ The report of the medical oflScer (education) to the London County Coun- 

 cil, 1909, contains 9 additional cades. 



