In the course of previous studies on the genetics of tooth formation and movement timing1' 2 we have encountered evidence suggestive of X-mediated inheritance.8-' These findings have raised the question of whether calcification, movement, and apical closure alone indicated sex-chromosomal involvement or whether tooth size and tooth morphology might also reveal evidence of X-chromosomal mediation.The present report, therefore, is concerned with tooth size in siblings of like-sex and unlike-sex. The mesiodistal tooth measurements were completed before the specific hypothesis was advanced for testing and therefore were free of unconscious bias in this respect.
Materials and MethodsThis study is based upon vernier micrometer caliper measurements of the mediodistal crown diameters of the permanent teeth of participants in the Fels Longitudinal Studies, as taken on plaster casts from alginate impressions. All measurements were read to the nearest 0.1 mm., transferred to pre-coding forms, and then to standard 80column IBM punch cards, especially coded to include family number, sex, and birth order. After verifying the punched data against the precoding forms, like-sex and cross-sex correlations were calculated for individual left and right and upper and lower teeth, respectively, for 73 subject pairs using the IBM 1620 Electronic computer at the Fels Computer Facility. Since all correlations were positive and since tests of significance of the differences between individual correlations were neither needed nor applicable, further calculations were not attempted except to provide weighted values of r from weighted mean values of z as described by R. A. Fisher.6
ResultsAs shown in Table 1, sibling similarities in permanent tooth size are unquestionable, with a mean value of r for all sibling combinations falling between 0.35 and 0.40, depending on the type of weighting employed.However, the mean value of r is deceptive, concealing as it does marked differences in the size of the correlations depending on whether like-sex or cross-sex comparisons are used. Thus sister-sister (SS) correlations in permanent tooth size are highest, varying from 0.46 to 0.82 for individual correlations and averaging 0.64 (using a weighted value of r). Brother-brother (BB) correlations are TABLE 1
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