SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 19 



or IV) do not agglutinate each other's blood, but the blood cor- 

 puscles of an individual represented by AA or Aa will be precipi- 

 tated if the donor contains the agglutinin represented by aa, and 

 conversely the blood corpuscles of an individual represented by 

 BB orBb will be precipitated if the donor contains the agglutinin 

 represented by bb. Inspection of the diagram will show that 

 group II (with serum bb) precipitates III and IV, and group III 

 (with serum aa ) precipitates II and IV. Further the serum of 

 group I (aa bb) precipitates all of the other groups; while the 

 serum of group IV precipitates none of the others. 



My fourth illustration has probably in some cases a glandular 

 basis, and in this sense has probably also a quantitative chemical 

 background. Height or stature in man is, in part, an hereditary 

 trait. It is sometimes said that short is dominant to tall, because 

 short parents may have both tall and short children, but tall 

 parents produce only tall children. This is probably an over- 

 statement, or at least a rather loose generalization. Height may 

 be due to long legs, or to a long body, or to a long neck or to time 

 of reaching maturity or to any combination of these; and these 

 differences may themselves be due to independent factors in in- 

 heritance. The best that we can do with height at present is to 

 refer it to a multiple factor basis, the actual factors being little 

 understood. 



In addition to these differences in stature, all of which we call 

 normal differences, there are certain extreme conditions superim- 

 posed on these as a background, in which the endocrine glands 

 probably play an important role. While it may well be that many 

 of these cases are caused by tumors of one of the glands, more 

 especially of the pituitary, thyroid, or testis, it is quite possible 

 that there may be actual inherited differences in the size and ac- 

 tivity of these glands. 



So far as I know there are no thoroughly worked out cases 

 of the inheritance of such differences in man or in mammals, but 

 in the case of certain races of birds I have been able to show both 

 by breeding tests and by castration experiments that glandular 

 differences are inherited according to the Mendelian scheme. 



