SEX DIFFERENCES IN THE EXPRESSION OF AUTOSOMAL 

 GENES AFFECTING HUMAN DENTITION 



F. B. HUTT 



University of Minnesota, University Farm, St. Paul, Minnesota 



During the past three years the writer has studied the genetic basis for 

 the occurrence of a gap between the upper central incisor teeth of man. 

 This condition is known to arise from an abnormal enlargement and attach- 

 ment of the frenum labium, a fold of mucous membrane which normally- 

 runs from the centre of the upper lip to the gum, where it is attached about 

 five millimetres above the gingiva. In persons exhibiting a gap between 

 the upper central incisors, it is usually found that this fold is enlarged to 

 form a sort of fibrous ligament which, instead of stopping above the teeth, 

 runs right between the central incisors and is attached behind them. 



In this study there were encountered a number of persons in which such a 

 space had arisen from some other cause and in whose pedigrees there was no 

 history of a similar condition. The majority of cases, however, present a 

 definite family history; and in the six pedigrees studied by the writer (and to 

 be published later) the peculiarity has behaved as a unifactorial autosomal 

 dominant character. 



However, in two of these pedigrees, and in two others not yet completed, 

 there occurred apparent exceptions to the rule in that parents reported to 

 have no space between the incisors had children in which such a space is 

 quite marked. In all four cases the apparently unaffected transmitter was 

 a mother with a family history of spaced central incisors. Two of these 

 could not be examined, one had incisors close together and the fourth was 

 found on examination to have a gap of about 1 mm. in width. In two other 

 cases girls reported not to show the peculiarity were later found to have 

 small gaps between the upper central teeth. Furthermore, in these six 

 pedigrees slightly fewer females than males had the gap; the figures for sib- 

 ships completely traced are given in table 1. 



This difference is not in itself significant, but all the findings together sug- 

 gested that the character was less common in women than in men. In order 

 to get data on the incidence of the condition in the general population an 

 arrangement was made whereby determination of its presence or absence 

 was made by competent dentists during their routine examination of 3,368 



447 



