316 M. H. RICHARDS AND RAY M. BALYEAT 



incapable of transmitting the gene for sensitivity if in our pedigrees it has 

 been entirely negative for two generations. 



The changing or incomplete dominance of allergy may best be explained 

 by quoting a couple of paragraphs from our earlier paper: 



The time of appearance of the sensitivity in the ontogeny of the individual differs 

 greatly in different people. In some cases a child develops a sensitivity very shortly, 

 after birth and in other cases individuals who have never before had trouble develop a 

 sensitivity at the age of seventy. Between these two extremes are a whole series of ages, 

 at any of which a sensitivity may develop. Consequently, if an individual belongs to an 

 affected family and has had no trouble, there is still a great probability that he may later 

 develop it. Thus no chart of an affected family can ever be considered as complete, for 

 those once classed as normal may later develop some form of allergy. 



Another point must also be considered. In order for any form of sensitivity to become 

 manifest there must be, in addition to the sensitivity, the external factor that causes the 

 trouble. A child sensitive to certain forms of protein will not develop urticaria or eczema 

 unless he eats that protein. Similarly an individual sensitive to a certain kind of pollen 

 develops hay fever only when exposed to that pollen. In recent years much has been 

 added to our knowledge of the interaction of heredity and environment. Apparently 

 the study of inheritance of hay fever, etc., reveals another case in which a definite environ- 

 ment as well as a definite inheritance is necessary to produce a definite character. 



These statements of our earlier paper apply equally well to migraine. 

 This form of allergy sometimes occurs in young children and on the other 

 hand many people from allergic families develop this difficulty much later 

 in life. The environment necessary to produce allergy in members of an 

 allergic family differs with respect to the different allergies. Hay fever 

 does not develop unless the patient comes in contact with a particular kind 

 of pollen which furnishes the necessary irritation. For example, a patient 

 born with the ability to become sensitive who comes in contact with Russian 

 thistle pollen develops a sensitivity to Russian thistle and may have hay 

 fever symptoms on adequate contact. Likewise, those who live in the 

 timothy section of the United States and who come in adequate contact 

 with timothy pollen have developed a sensitivity to timothy and have hay 

 fever symptoms from contact with timothy pollen. Similarly migraine, 

 eczema, etc. develop only when the foods to which the individual is sensitive 

 are eaten. Both the pollen in the one case and the particular food in the 

 other may be regarded as the environment necessary to bring out the allergy. 



The data upon which these conclusions in regard to migraine are based 

 are less open to criticism than in the case of hay fever on the score of the 

 environmental condition necessary to bring out the sensibility. That is, 

 there is less possibility that the negative cases are not truly negative, but 

 have merely not happened to meet the proper environment to cause the 



