252 GENETICS 



culture medium for the bacteria of tuberculosis than are 

 others. A person who gets such a combination of genes will 

 develop tuberculosis, if he lives In a region in which the 

 germs of that disease are abundant; while another person, 

 with a different set of genes, will not be subject to tuber- 

 culosis, even though living under the same conditions. And 

 the person with the genes that make him susceptible to 

 tuberculosis will not have the disease if he prevents infec- 

 tion by the tubercle bacillus. In relation to such susceptibil- 

 ity there are doubtless a great number of grades, dependent 

 on just what combination of genes is present. Some are 

 extremely susceptible, others less so but still prone to the 

 disease, and so on, up to individuals whose gene combina- 

 tions are such as to give them natural immunity to tuber- 

 culosis. 



What then shall be answered when it is asked whether 

 tuberculosis is hereditary or not? One can only say that 

 some gene combinations predispose to it more than others, 

 so that a hereditary factor is involved. But also there is a 

 necessary environmental factor; it is the interrelation of the 

 two sets of factors that determines whether the individual 

 shall be afflicted with the disease. 



Similar considerations apply to any other disease, acute 

 or chronic. Certain combinations of genes are doubtless 

 more readily attacked by plague, by smallpox, by typhoid, 

 by pneumonia, than others, just as certain genetic constitu- 

 tions yield more readily to extremes of temperature, to ex- 

 posure to the elements, to unfit food. There is no affair of 

 life in which the gene combination borne by the individual 

 does not play a part. But the environment too plays a part, 

 often an overwhelmingly important part. 



The relative role played by genes on the one hand, and 

 by environment on the other, is not the same for all charac- 

 teristics. Differences in certain features of the organism are 



