Heredity 2 1 9 



The studies of human material from the pathological standpoint show, 

 as has been said, that most individuals become infected with tuberculosis 

 at one time or another and it may therefore be concluded that neither in 

 kind nor degree are the inherited factors capable of preventing infection. 

 They must, therefore, be exerted on the progress of the disease after the 

 body is invaded by Bacilhis tuberculosis * The direct evidence at present 

 available from human sources does not carry us beyond this point. 



In the guinea pig it is found that there are inherited factors which in- 

 fluence the quantity of antibodies (antitoxic substances) which are pro- 

 duced in response to a given stimulus. There are other inherited factors 

 which influence the severity and precise quality of the ulceration which 

 the tubercle bacillus and some other irritating agents produce in the skin, 

 and in the character of the tuberculous inflammation in the lymphatic 

 vessels and glands. There is also an indication of another group of sepa- 

 rately inherited factors affecting the nature of the reaction to dietary de- 

 ficiencies. 



Granted that there are inheritable factors influencing the character of 

 tuberculosis in the individual, any clue as to their dominant or recessive 

 quality is a matter of great interest. Unfortunately the human material 

 lacks the precision of detail necessary for an answer to such a question. 

 The guinea-pig material suggests that where all of the characters favor- 

 able to resistance are combined in a family it presents a dominant com- 

 bination. The first generation crossbreds are as resistant as the most re- 

 sistant family. In the actual observations they somewhat surpass this mark, 

 indicating the operation of those forces which make for heterosis or hybrid 

 vigor. Where crosses are made between families of less than the maximum 

 resistance the result varies. Some crosses produce offspring as resistant as 

 the better family, another produces an intermediate resistance. In gen- 

 eral, dominance of resistance prevails but it is imperfect. 



With the coincident and tremendous improvement in hygienic condi- 

 tions and nutritional well-being in Europe and especially in America, 

 tuberculosis and the minor infections referred to have a greatly diminished 

 prevalence. 



A generation ago the general conception of the fundamental nature of 

 inheritance was that it was a blending or fusing of the parental character- 

 istics, stronger characters being diluted by weaker. The cases which such 

 a blend did not explain were regarded as unaccountable exceptions. Then 

 the work of Mendel was revived and it was seen that when inherited 

 qualities were sufficiently analyzed into their component parts the blended 

 was rather the exceptional occurrence. But instances of blending inherit- 

 ance could not be gotten over or disregarded and it seemed to some 

 students that there must be two principal forms of inheritance. These 

 views have been quite completely harmonized by further study. In the 



• Now known as Mycobacterium tuberculosis. — Ed. 



