TWIN STUDY OF DENTAL DIMENSION .1. DISCORDANCE, ASYMMETRY, AND MIRROR IMAGERY

Bilateral tooth measurements in twins were partitioned into 3 orthogonal contrasts, each associated with 1 degree of freedom, to estimate 3 parameters: discordance, asymmetry and mirror imagery. The probability levels of the within-pair variance ratios were used to test for significance of these estimates. There may exist significant genetic determinants of almost all of the individual tooth dimensions, but there is little or no evidence for a genetic basis of asymmetry. The analysis gave no indication that monozygotic twinning was associated with an increased degree of fluctuating asymmetry or mirror imagery, when compared to dizygotic twins. The data on monozygotic twins suggested that for most variables examined, the increment of environmental discordance resulting from the twinning phenomena was greater than the developmental noise that caused asymmetry within individual cotwins.