THE INHERITANCE OF DISEASE 5O I 



But skin color among the pigmented of the human species, 

 tallness or shortness in the human race (excepting particular 

 types of dwarfism), the weight or ear length in rabbits and 

 innumerable other conditions are at first sight not so con- 

 trolled. The result of a cross between individuals of widely 

 different character is usually a "blended" or intermediate 

 state in the offspring. While it was difficult at first, as has 

 been said, to fit these cases to the Mendehan hypothesis it is 

 now apparent that blended inheritance means that the 

 character as expressed in the individual is the resultant of 

 the combined and overlapping functional expression of the 

 action of two or more genes. It now is the consensus of 

 opinion among students of heredity that this is the true 

 significance of blended inheritance. Mendehan principles are 

 as strictly apphcable as in the more obvious instances but 

 more than one, often many, unit characters are involved 

 in the make-up of the observable quality. This is evidently 

 the condition underlying the inherited factors in resistance 

 to tuberculosis. 



The guinea-pig material submitted to analysis with this 

 principle in mind gives the following provisional result: 

 For each of the five famihes there is a characteristic grade of 

 resistance. This could be accounted for by the action of two 

 separately inherited unit characters but not by one. The 

 study of the crosses between the families indicates that 

 there are operative not less than three and possibly four unit 

 characters. The study of the physiological reactions shows 

 suggestive relations in such widely separated functional 

 activities as the immunological reactions, the tissue reactions, 

 and responses to dietary changes, with an observable inde- 

 pendence between them. This would also justify the assump- 

 tion of at least three characters. The tissue reactions when 

 further analyzed are found to be complex, involving at the 

 least two characters. The other types of reaction are obvi- 

 ously blended and must involve at least two characters. The 

 results at hand then must be assumed to involve at least six 

 and possibly eight separately inherited unit characters. 

 Probably the matter is much more comphcated than this in 

 the guinea pig, and even more so in the human. 



