318 M. H. RICHARDS AND RAY M. BALYEAT 



allergic and a special modifying gene being necessary to produce each type 

 of allergy. This possibility mentioned in our previous paper, has been 

 suggested by several critics, but the number of pedigrees previously con- 

 sidered was insufficient to settle the point. 



In order to gain some light on this matter we have examined carefully 

 the pedigrees of those families in which an allergic individual marries into 

 a negative line. As already stated, a line is considered negative when for 

 two generations no member has exhibited an allergy. The study of this 

 type of mating has proven the most productive phase of our research. 

 Obviously a cross of this kind will show at once what types of allergy are 

 dominant, for if any allergy appears again in the F x from such a cross, its 

 dominance is certain. 



Unfortunately, as we have stated before, marriages of this type are not 

 numerous. We have among our fifty-three pedigrees twelve matings of this 

 type (Nos. 1, 5, 12, 18, 29, 32, 34, 37, 38, 41, 42, 43). In pedigree 1, hay 

 fever and migraine act as dominants; in No. 12, hay fever; in No. 29, urti- 

 caria and migraine; in No. 34, eczema; and in No. 43, asthma acts in a 

 similar manner. Thus, each of these five types of allergy may show 

 dominance. 



These marriages between allergic individuals and persons from negative 

 lines reveal several definite points. In spite of the dominance of the differ- 

 ent forms of allergy, not even half of the Fi which we should expect, if the 

 allergic parent were heterozygous, develop the sensitivity. 



Another fact revealed by this type of mating is that a person with one 

 form of allergy may transmit other forms to his or her children. Examples 

 to illustrate this condition are to be found in many pedigrees and are of two 

 kinds : Either the allergy not present in the parent may have been observed 

 in other members of his family, or in other cases some form entirely new 

 to the strain may appear. 



A summary of the results of some of the matings between allergic and 

 negative families will make these facts more evident (see table 1). 



If we consider sensitivity as due to one gene and each allergy as due to 

 a modifying gene, then we must assume that in each of these cases the 

 negative line brought in the special modifiers necessary to produce the 

 different types of allergy. If these modifiers exist, it has just been shown 

 that in connection with the gene for sensitivity they must act as dominants. 

 Since these possible modifiers are dominant, the negative lines referred to 

 above would have to bring in one or more dominant modifying genes for 

 the different forms of allergy, these modifying genes being unable to express 

 themselves without the gene for sensitivity which is lacking in the negative 



