SEX DIFFERENCES IN HUMAN DENTITION 451 



puted the following frequencies of the condition which Downs designates as 

 "spaced uppers" and which is probably wholly or partly the same as that 

 considered in this paper: 



per cent 



In 375 persons with endocrine disorders 12.27 ± 1.68 



In 271 normal persons 2.58 ± 1.04 



Difference 9.69 ± 1.97 



Since this difference is nearly five times its standard error, it indicates 

 that the greater frequency of spaced uppers in persons with endocrine dis- 

 orders is significant. Unfortunately Downs did not give the distribution 

 of these cases in the two sexes. 



It is probably a coincidence that an endocrine basis for sex-limitation of 

 an autosomal gene is found in two other characters affecting the dentition 

 of man. One of these is found in the pedigree of female-sex-limited prema- 

 ture decay and loss of teeth reported by Sedgwick (cited by Gates, 1929) 

 in which 10 of 11 daughters in two generations were affected while none of 

 four sons showed the character. A similar tendency is evident in a pedigree 

 of female-sex-limited missing lateral incisors given by Schultz (1932). This 

 last is of particular interest because the character has been found in other 

 pedigrees to be very irregular in its expression. Adequate data on its sex 

 distribution are not available but the influence of sex on its expression is 

 definitely shown in Schultz's pedigree and it may well be a factor in causing 

 the other irregularities. 



It is desirable to point out that while a male-sex-limited character, such 

 as is found in Schofield's (1921) pedigree of webbed toes, can be accounted 

 for apart from endocrines by postulating a gene in the Y-chromosome, a 

 female-sex-limited character such as the two just cited can not be adequately 

 accounted for by sex-linked genes, since in the offspring of an affected 

 female the character would appear in both males and females were it not for 

 the modifying influence of the endocrine secretions. 



The hereditary basis for spaced upper incisors is apparently intermediate 

 between that of a simple dominant character appearing equally in both sexes 

 and that for such a character as pattern baldness which, from the studies 

 of Osborn (1916), would appear to be manifested in males when they are 

 heterozygous for it, but in females only when they are homozygous. In the 

 latter case a single gene can be fully expressed in males but not in females, 

 where the duplex condition is necessary to overcome the inhibiting forces of 

 the endocrine environment. 



These data on spaced incisors are presented to show that sex differences in 

 the expression of an autosomal character may obscure its genetic basis un- 



