Variability of Postnatal Ossification Timing and Evidence

Although absolute variability in postnatal ossification timing is generaIly larger in boys than in girls, reIative, conception-corrected variability is signscantly larger in girls, suggestive of a “dosage” effect and consistent with the hypothesis of partial X-linkage. These findings, together with the excess of sister-sister over brother-brother timing similarities are inconsistent with the hypothesis of selective inactivation of either the paternal or the maternal X chromosome. In previous studies on postnatal ossification timing, we have explored various aspects of genetic control, as studied both within generation and between them. The reports have emphasized the familial nature of “atypical” ossification sequences and sequence polymorphisms (Garn and Rohmann, ’60, ’62b; Garn, Rohmann and Davis, ’63; Garn, Rohmann and Blumenthal, ’66; Garn, Rohmann and Silverman, ’67) and the evidence for both autosomal and sex-linked inheritance (Garn and Rohmann, ’62a, ’69). Both the studies of sequence variability and sequence polymorphism and the investigations on the mechanisms of control have indicated the need for a comprehensive analysis of variability in ossification timing, center by center, in part to test for a “dosage effect,” and in part to relate sequence variability to ossification timing variability per se. The present paper, therefore, is concerned with three aspects of variability in postnatal ossification timing. It is first concerned with the sex difference in absolute variability of 73 postnatal centers, with particular reference to the late appearing “adolescent” centers. Second, it is concerned with the sex difference in relative or age-corrected variability, with specific reference to possibly enhanced relative variability in the female, consistent with the hypothesis of partial mediation of the X chromosome. Third, it is concerned with the implications of ossification timing variability to sequence variability and sequence polymorphisms, to missing secondary centers of ossification and to the comAM. J. PHYC. ANTHROP.. 32: 139-144. munality of ossification timing and the predictive efficiency of different centers of ossification. MATERIALS AND METHODS This study is based upon cumulative ageat-appearance data on 73 postnatal ossification centers of the hand, foot, elbow, knee, shoulder and hip, and including epiphyses, round bones and primary centers. The fifteenth and eighty-fifth percentiles were recalculated from the data earlier employed by us (Garn, Rohmann and Silverman, ’67), and were then used to estimate ossification timing variability (i.e. 62) center by center, and for boys and girls respectively. For each center, and for boys and girls separately, relative variance was also calculated as the familiar coefficient of variation (C. V.) thus relating absolute variability in postnatal ossification timing to the conception-corrected median age at appearance, for the center and sex in question. The familiar F test was employed to determine the significance of sex differences in ossification timing variance, and the t test was employed to compare pooled data on both absolute and relative variance. With 73 postnatal centers, the stochastic chi-squared test could also be applied without the need for Yates’ correction for continuity. The primary question was whether and to what extent relative variability in postnatal ossification timing was greater in the female than in the male, consistent with the hypothesis of partial X-linkage, and