HEREDITY IN MAN 367 



serious but none the U^ss unfortunate (h^fects of deafness, tendency 

 to disease and similar conditions is illustrated by a large numl^er 

 of recorded pedigrees. Kellicott reproduces several from Whet- 

 ham's Treasury of Human Inheritance, including the following 

 record of inherited deaf-mutism (Fig. 202). This case is ade- 

 quately illustrated in the diagram, but attention should be given 

 especialh" to the frequent marriage of defectives in this line and 

 to the fact that even normal unions produced defective offspring. 

 In contrast to the unavoidable state of deaf-mutism other 

 pedigrees show the constant recurrence of tuberculosis. This 

 disease must be acquired by every individual. Possibly nobody 



1 no 



u m® mm m® ? ao ^d 



mm® ? ■•■••DAAAAAD? •no«B«DAA 2m®m®m®m® 





V AB ? • ?B«*B«»OAA AB B 



Fig. 202. — A family hi.story showing the inheritance of deaf-muti.sm. Males 



are indicated by squares, females by circles, and individuals of unknown 



sex by triangles; deafness is shown by black, normal hearing by white, and 



uncertain data by shaded areas. (Modified from Kellicott, after Whetham.) 



goes through life without infection but some persons lack the 

 inherent qualities to resist the attack of the bacillus and so 

 develop tuberculosis. The recognition of such an inherited weak- 

 ness of resistence, or as we usually say, tendency to the disease, 

 should be a warning which would lead to the proper steps for 

 avoiding its serious results. Choice of location, occupation, and 

 recreation might well offset the inherent defect. 



Inbreeding. The preceding cases are very good illustrations 

 of the effects of inbreeding in the human race. Defective lines, 

 whether the defect is dominant or recessive, are improved by 

 intermarriage with normal lines. Since recessive defects are not 

 evident in heterozygous individuals it is impossible to predict 

 what the results of marriage of normal individuals may be within 

 lines known to have such defects. These marriages are very 



