Disease and Heredity 319 



It is assumed that the trait in question is actually a 

 modification — shortsightedness, for example, or bleeding 

 (haemophilia, see page 264) , or hornlessness in cattle. A man 

 has a severe nosebleed — almost dies, but not quite; it is a 

 dramatic episode and not easily forgotten. The circum- 

 stances leading to the nosebleed are remembered. Is it any 

 wonder that, when years afterward his grandson repeats the 

 experience, people will recall and point to the transmitted 

 effect of a severe blow on the nose? It happens that de- 

 fective clotting of blood is a physiological condition the 

 iriheritance of which has been pretty thoroughly studied. 

 The conditions appear to be a " sex-linked recessive " and 

 one could predict the manifestation of the trait in a fam- 

 ily of known genetic composition, except for the circum- 

 stance that human families are ordinarily not large enough 

 to permit of statistical treatment. It happens also that 

 hornlessness in cattle has appeared as a mutation (not a 

 mutilation) and reappears in heredity as a Mendelian 

 dominant. 



The other assumption that invalidates many of the il- 

 lustrations of the supposed law is that the reappearance of a 

 modification is necessarily evidence of its inheritance. Lon- 

 gevity may be, probably is, the result of numerous inherited 

 factors. Being killed by an automobile at forty years of 

 age is probably not, althoug:h there are cases in which father 

 and son terminated life in similar fashion at about the same 

 age. A large class of recurrent modifications is found in 

 the repeated appearance of an infectious disease. For cen- 

 turies it was common to see pock-marked faces; and then 

 such faces would be borne by all the members of a family. 

 The modification, however, was a fresh one for each in- 

 dividual. Other effects of disease would fall in the same class. 



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Disease and Heredity 



The fact that one or another disease " runs in a family " 

 has also given rise to misunderstandings. The physician to- 



