450 F. B. HTJTT 



Lenz, 1931). The elimination of the propositus does not entail any great 

 error since the chance of the character appearing in any brother or sister was 

 as great as that of its appearing in the discarded propositus. The method 

 does reduce the frequency of the condition because in many families the 

 numbers were too small to obtain a representative ratio after the elimina- 

 tion of the propositus. This is to some extent compensated for when a 

 considerable number of sibships are considered. 



In this case the sibships yielded, as was expected, a normal sex ratio of 

 86a 71 cf to 83 9 9 , but only 38.5 per cent of the females were affected in con- 

 trast to the figure of 46.5 per cent for males. Since this group may possibly 

 have included some cases of non-genetic spaced teeth, there are considered 

 in class B4 the sibships of these students having definite evidence of a hered- 

 itary gap in the form of an affected parent. In this group the sex difference 

 is much more pronounced. 



Actual measurements of the gap have as yet been made on only a small 

 number of persons but in these the space is somewhat smaller in males than 

 in females. (See table 4.) 



DISCUSSION 



In consideration of the evidence from these several angles it is consistent 

 with the data to conclude that the space between the upper central incisors 

 caused by hereditary abnormality of the frenum labium is sometimes sup- 

 pressed in whole or in part in females. 



On this basis one can account for the pedigrees in which the character 

 appears to have skipped a generation, i.e., to be a recessive. On the other 

 hand one must recognize that such suppression or modification is not found 

 in every case. This is attested by the fact that the widest gap thus far en- 

 countered, 4.5 mm., was in a girl. Presumably there are modifiers for this 

 gene as for others. The degree of expression of the character must depend 

 upon the interaction of the causative gene with its modifiers and its environ- 

 ment. The sex difference in its expression may be considered the result of 

 differential responses of the causative gene and its modifiers to the different 

 endocrine environments found in the two sexes. 



In this connection the findings of Downs (1927) are of special interest. 

 In studying the occurrence of various abnormalities of dentition in com- 

 bination with sixteen different pathological conditions indicative of endo- 

 crine dyscrasias, no association was found between any specific dental 

 anomaly and any specific endocrine disorder but abnormalities in dentition 

 were approximately three times as frequent in those with endocrine dys- 

 crasia as in normal persons. From these data the present writer has com- 



